remove let-env, focus on mutating $env (#9574)

# Description

For years, Nushell has used `let-env` to set a single environment
variable. As our work on scoping continued, we refined what it meant for
a variable to be in scope using `let` but never updated how `let-env`
would work. Instead, `let-env` confusingly created mutations to the
command's copy of `$env`.

So, to help fix the mental model and point people to the right way of
thinking about what changing the environment means, this PR removes
`let-env` to encourage people to think of it as updating the command's
environment variable via mutation.

Before:

```
let-env FOO = "BAR"
```

Now:

```
$env.FOO = "BAR"
```

It's also a good reminder that the environment owned by the command is
in the `$env` variable rather than global like it is in other shells.

# User-Facing Changes

BREAKING CHANGE BREAKING CHANGE

This completely removes `let-env FOO = "BAR"` so that we can focus on
`$env.FOO = "BAR"`.

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After / Before Submitting
integration scripts to update:
- ✔️
[starship](https://github.com/starship/starship/blob/master/src/init/starship.nu)
- ✔️
[virtualenv](https://github.com/pypa/virtualenv/blob/main/src/virtualenv/activation/nushell/activate.nu)
- ✔️
[atuin](https://github.com/ellie/atuin/blob/main/atuin/src/shell/atuin.nu)
(PR: https://github.com/ellie/atuin/pull/1080)
- ❌
[zoxide](https://github.com/ajeetdsouza/zoxide/blob/main/templates/nushell.txt)
(PR: https://github.com/ajeetdsouza/zoxide/pull/587)
- ✔️
[oh-my-posh](https://github.com/JanDeDobbeleer/oh-my-posh/blob/main/src/shell/scripts/omp.nu)
(pr: https://github.com/JanDeDobbeleer/oh-my-posh/pull/4011)

crates/nu-utils/src/sample_config/default_config.nu

@@ -183,7 +183,7 @@ let light_theme = {
 
 
 # The default config record. This is where much of your global configuration is setup.
-let-env config = {
+$env.config = {
   # true or false to enable or disable the welcome banner at startup
   show_banner: true
   ls: {

crates/nu-utils/src/sample_config/default_env.nu

@@ -44,21 +44,21 @@ def create_right_prompt [] {
 }
 
 # Use nushell functions to define your right and left prompt
-let-env PROMPT_COMMAND = {|| create_left_prompt }
-let-env PROMPT_COMMAND_RIGHT = {|| create_right_prompt }
+$env.PROMPT_COMMAND = {|| create_left_prompt }
+$env.PROMPT_COMMAND_RIGHT = {|| create_right_prompt }
 
 # The prompt indicators are environmental variables that represent
 # the state of the prompt
-let-env PROMPT_INDICATOR = {|| "> " }
-let-env PROMPT_INDICATOR_VI_INSERT = {|| ": " }
-let-env PROMPT_INDICATOR_VI_NORMAL = {|| "> " }
-let-env PROMPT_MULTILINE_INDICATOR = {|| "::: " }
+$env.PROMPT_INDICATOR = {|| "> " }
+$env.PROMPT_INDICATOR_VI_INSERT = {|| ": " }
+$env.PROMPT_INDICATOR_VI_NORMAL = {|| "> " }
+$env.PROMPT_MULTILINE_INDICATOR = {|| "::: " }
 
 # Specifies how environment variables are:
 # - converted from a string to a value on Nushell startup (from_string)
 # - converted from a value back to a string when running external commands (to_string)
 # Note: The conversions happen *after* config.nu is loaded
-let-env ENV_CONVERSIONS = {
+$env.ENV_CONVERSIONS = {
   "PATH": {
     from_string: { |s| $s | split row (char esep) | path expand --no-symlink }
     to_string: { |v| $v | path expand --no-symlink | str join (char esep) }
@@ -72,16 +72,16 @@ let-env ENV_CONVERSIONS = {
 # Directories to search for scripts when calling source or use
 #
 # By default, <nushell-config-dir>/scripts is added
-let-env NU_LIB_DIRS = [
+$env.NU_LIB_DIRS = [
     ($nu.default-config-dir | path join 'scripts')
 ]
 
 # Directories to search for plugin binaries when calling register
 #
 # By default, <nushell-config-dir>/plugins is added
-let-env NU_PLUGIN_DIRS = [
+$env.NU_PLUGIN_DIRS = [
     ($nu.default-config-dir | path join 'plugins')
 ]
 
 # To add entries to PATH (on Windows you might use Path), you can use the following pattern:
-# let-env PATH = ($env.PATH | split row (char esep) | prepend '/some/path')
+# $env.PATH = ($env.PATH | split row (char esep) | prepend '/some/path')

crates/nu-utils/src/sample_config/sample_login.nu

@@ -3,7 +3,7 @@
 # - will be sourced after config.nu and env.nu in case of nushell started as login shell
 
 # just as an example for overwriting of an environment variable of env.nu
-let-env PROMPT_INDICATOR = {|| "(LS)> " }
+$env.PROMPT_INDICATOR = {|| "(LS)> " }
 
 # Similar to env-path and config-path there is a variable containing the path to login.nu
 echo $nu.loginshell-path