Always load default env/config values (#14249)

# Release-Notes Short Description

* Nushell now always loads its internal `default_env.nu` before the user
`env.nu` is loaded, then loads the internal `default_config.nu` before
the user's `config.nu` is loaded. This allows for a simpler
user-configuration experience. The Configuration Chapter of the Book
will be updated soon with the new behavior.

# Description

Implements the main ideas in #13671 and a few more:

* Users can now specify only the environment and config options they
want to override in *their* `env.nu` and `config.nu`and yet still have
access to all of the defaults:
* `default_env.nu` (internally defined) will be loaded whenever (and
before) the user's `env.nu` is loaded.
* `default_config.nu` (internally defined) will be loaded whenever (and
before) the user's `config.nu` is loaded.
* No more 900+ line config out-of-the-box.
* Faster startup (again): ~40-45% improvement in launch time with a
default configuration.
* New keys that are added to the defaults in the future will
automatically be available to all users after updating Nushell. No need
to regenerate config to get the new defaults.
* It is now possible to have different internal defaults (which will be
used with `-c` and scripts) vs. REPL defaults. This would have solved
many of the user complaints about the [`display_errors`
implementation](https://www.nushell.sh/blog/2024-09-17-nushell_0_98_0.html#non-zero-exit-codes-are-now-errors-toc).
* A basic "scaffold" `config.nu` and `env.nu` are created on first
launch (if the config directory isn't present).
* Improved "out-of-the-box" experience (OOBE) - No longer asks to create
the files; the minimal scaffolding will be automatically created. If
deleted, they will not be regenerated. This provides a better
"out-of-the-box" experience for the user as they no longer have to make
this decision (without much info on the pros or cons) when first
launching.
* <s>(New: 2024-11-07) Runs the env_conversions process after the
`default_env.nu` is loaded so that users can treat `Path`/`PATH` as
lists in their own config.</s>
* (New: 2024-11-08) Given the changes in #13802, `default_config.nu`
will be a minimal file to minimize load-times. This shaves another (on
my system) ~3ms off the base launch time.
* Related: Keybindings, menus, and hooks that are already internal
defaults are no longer duplicated in `$env.config`. The documentation
will be updated to cover these scenarios.
* (New: 2024-11-08) Move existing "full" `default_config.nu` to
`sample_config.nu` for short-term "documentation" purposes.
* (New: 2024-11-18) Move the `dark-theme` and `light-theme` to Standard
Library and demonstrate their use - Also improves startup times, but
we're reaching the limit of optimization.
* (New: 2024-11-18) Extensively documented/commented `sample_env.nu` and
`sample_config.nu`. These can be displayed in-shell using (for example)
`config nu --sample | nu-highlight | less -R`. Note: Much of this will
eventually be moved to or (some) duplicated in the Doc. But for now,
this some nice in-shell doc that replaces the older
"commented/documented default".
* (New: 2024-11-20) Runs the `ENV_CONVERSIONS` process (1) after the
`default_env.nu` (allows `PATH` to be used as a list in user's `env.nu`)
and (2) before `default_config.nu` is loaded (allows user's
`ENV_CONVERSIONS` from their `env.nu` to be used in their `config.nu`).
* <s>(New: 2024-11-20) The default `ENV_CONVERSIONS` is now an empty
record. The internal Rust code handles `PATH` (and variants) conversions
regardless of the `ENV_CONVERSIONS` variable. This shaves a *very* small
amount of time off the startup.</s> Reset - Looks like there might be a
bug in `nu-enginer::env::ensure_path()` on Windows that would need to be
fixed in order for this to work.

# User-Facing Changes

By default, you shouldn't see much, if any, change when running this
with your existing configuration.

To see the greatest benefit from these changes, you'll probably want to
start with a "fresh" config. This can be easily tested using something
like:

```nushell
let temp_home = (mktemp -d)
$env.XDG_CONFIG_HOME = $temp_home
$env.XDG_DATA_HOME = $temp_home
./target/release/nu
```

You should see a message where the (mostly empty) `env.nu` and
`config.nu` are created on first start. Defaults should be the same (or
similar to) those before the PR. Please let me know if you notice any
differences.

---

Users should now specify configuration in terms of overrides of each
setting. For instance, rather than modifying `history` settings in the
monolithic `config.nu`, the following is recommended in an updated
`config.nu`:

```nu
$env.config.history = {
  file_format: sqlite,
  sync_on_enter: true
  isolation: true
  max_size: 1_000_000
}
```

or even just:

```nu
$env.config.history.file_format = sqlite
$env.config.history.isolation: true
$env.config.history.max_size = 1_000_000
```

Note: It seems many users are already appending a `source my_config.nu`
(or similar pattern) to the end of the existing `config.nu` to make
updates easier. In this case, they will likely want to remove all of the
previous defaults and just move their `my_config.nu` to `config.nu`.

Note: It should be unlikely that there are any breaking changes here,
but there's a slim chance that some code, somewhere, *expects* an
absence of certain config values. Otherwise, all config values are
available before and after this change.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

Configuration Chapter (and related) of the doc is currently WIP and will
be finished in time for 0.101 release.

crates/nu-utils/src/default_files/README.md

@@ -0,0 +1,82 @@
+# Nushell configuration files
+
+## `default_env.nu`:
+
+* The internal default environment variables (other than `$env.config`) that will be set during Nushell startup.
+* Is loaded *before* the user's `env.nu`.
+* Will be loaded during any startup where the user's `env.nu` is also loaded. For example:
+  * During normal startup with `nu`
+  * During a startup where the user specifies an alternative `env.nu` via `nu --env-config <path>`
+* Likewise, is never loaded during a startup where the user's `env.nu` would not be loaded. For example:
+  * `nu -n/--no-config`
+  * `nu -c "ls"`
+  * `nu <script.nu>`
+* Is not commented - Comments are in `sample_env.nu`.
+* Should be optimized for fastest load times.
+* Can be introspected via `config env --default | nu-highlight`
+
+## `default_config.nu`:
+
+Counterpart to `default_env.nu`.
+
+* Contains any `$env.config` values that are not set via Rust defaults.
+* Is loaded *after* the user's `env.nu`.
+* Is loaded *before* the user's `config.nu`.
+* Will be loaded during any startup where the user's `config.nu` is also loaded. For example:
+  * During normal startup with `nu`
+  * During a startup where the user specifies an alternative `config.nu` via `nu --config <path>`
+* Likewise, is never loaded during a startup where the user's `config.nu` would not be loaded. For example:
+  * `nu -n/--no-config`
+  * `nu -c "ls"`
+  * `nu <script.nu>`
+* Is not commented - Comments are in `sample_config.nu`.
+* Should be optimized for fastest load times. Whenever possible, values should be set via nu-protocol::config
+  * Exception: `color_config` values are currently set in this file so that user's can introspect the values
+  * TODO: Implement defaults for `color_config` in nu-protocol::config and remove from `default_config.nu`
+* Can be introspected via `config nu --default | nu-highlight`
+* An ideal `default_config.nu` (when all values are set via `nu-protocol::config`) will simply be:
+  ```
+  $env.config = {}
+  ```
+
+## `sample_env.nu`
+
+* A commented file documenting the most common environment variables that a user might configure in `env.nu`
+* For convenient in-shell access
+* Can be pretty-printed via `config env --sample | nu-highlight`
+* Since this file is for documentation only, include actual Nushell code without comments so that it can be pretty-printed
+* No optimization necessary - Not intended for use other than documentation.
+* Consider replacing `config env --sample` with `help env.nu` at some point.
+* Uses a mix of default values (explained) as well as other examples that users might want in their own `env.nu`
+
+## `sample_config.nu`
+
+Counterpart to `sample_env.nu`.
+
+TODO: **Not in final form**
+
+* A commented file documenting the most common environment variables that a user might configure in `config.nu`
+* For convenient in-shell access
+* Can be pretty-printed via `config nu --sample | nu-highlight`
+* Since this file is for documentation only, include actual Nushell code without comments so that it can be pretty-printed
+* No optimization necessary - Not intended for use other than documentation.
+* Consider replacing `config nu --sample` with `help config.nu` at some point.
+* Uses a mix of default values (explained) as well as other examples that users might want in their own `config.nu`
+
+## `scaffold_env.nu`
+
+* This file is used *one-time* (typically) at **first** startup
+* If the `$nu.default-config-path` directory does not exist, the directory is created and then both `scaffold_env.nu` and `scaffold_config.nu` are written to it
+* Contains only commented lines explaining the purpose of the file to the user, along with information on the `config env` command.
+
+## `scaffold_config.nu`
+
+Counterpart to `scaffold_env.nu`.
+
+* This file is used *one-time* (typically) at **first** startup
+* If the `$nu.default-config-path` directory does not exist, the directory is created and then both `scaffold_env.nu` and `scaffold_config.nu` are written to it
+* Contains only commented lines explaining the purpose of the file to the user, along with information on the `config nu` command.
+
+## `sample_login.nu`
+
+This file is not used by any Nushell code. Of course, if the user has a `login.nu`, then it will be evaluated during startup of a login shell.