Convert Path to list in main and preserve case (#14764)

# Description

Fixes multiple issues related to `ENV_CONVERSION` and
path-conversion-to-list.

* #14681 removed some calls to `convert_env_values()`, but we found that
this caused `nu -n` to no longer convert the path properly.
* `ENV_CONVERSIONS` have apparently never preserved case, meaning a
conversion with a key of `foo` would not update `$env.FOO` but rather
create a new environment variable with a different case.
* There was a partial code-path that attempted to solve this for `PATH`,
but it only worked for `PATH` and `Path`.
* `convert_env_values()`, which handled `ENV_CONVERSIONS` was called in
multiple places in the startup depending on flags.

This PR:

* Refactors the startup to handle the conversion in `main()` rather than
in each potential startup path
* Updates `get_env_var_insensitive()` functions added in #14390 to
return the name of the environment variable with its original case. This
allows code that updates environment variables to preserve the case.
* Makes use of the updated function in `ENV_CONVERSIONS` to preserve the
case of any updated environment variables. The `ENV_CONVERSION` key
itself is still case **insensitive**.
* Makes use of the updated function to preserve the case of the `PATH`
environment variable (normally handled separately, regardless of whether
or not there was an `ENV_CONVERSION` for it).

## Before

`env_convert_values` was run:

* Before the user `env.nu` ran, which included `nu -c <commandstring>`
and `nu <script.nu>`
* Before the REPL loaded, which included `nu -n`

## After

`env_convert_values` always runs once in `main()` before any config file
is processed or the REPL is started

# User-Facing Changes

Bug fixes

# Tests + Formatting

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

Added additional tests to prevent future regression.

# After Submitting

There is additional cleanup that should probably be done in
`convert_env_values()`. This function previously handled
`ENV_CONVERSIONS`, but there is no longer any need for this since
`convert_env_vars()` runs whenever `$env.ENV_CONVERSIONS` changes now.

This means that the only relevant task in the old `convert_env_values()`
is to convert the `PATH` to a list, and ensure that it is a list of
strings. It's still calling the `from_string` conversion on every
variable (just once) even though there are no `ENV_CONVERSIONS` at this
point.

Leaving that to another PR though, while we get the core issue fixed
with this one.

src/config_files.rs

@@ -2,14 +2,13 @@ use log::warn;
 #[cfg(feature = "plugin")]
 use nu_cli::read_plugin_file;
 use nu_cli::{eval_config_contents, eval_source};
-use nu_engine::convert_env_values;
 use nu_path::canonicalize_with;
 use nu_protocol::{
     engine::{EngineState, Stack, StateWorkingSet},
     eval_const::{get_user_autoload_dirs, get_vendor_autoload_dirs},
     report_parse_error, report_shell_error, Config, ParseError, PipelineData, Spanned,
 };
-use nu_utils::{get_default_config, get_default_env, get_scaffold_config, get_scaffold_env, perf};
+use nu_utils::{get_default_config, get_default_env, get_scaffold_config, get_scaffold_env};
 use std::{
     fs,
     fs::File,
@@ -37,20 +36,6 @@ pub(crate) fn read_config_file(
 
     if is_env_config {
         eval_default_config(engine_state, stack, get_default_env(), is_env_config);
-
-        let start_time = std::time::Instant::now();
-        let config = engine_state.get_config();
-        let use_color = config.use_ansi_coloring.get(engine_state);
-        // Translate environment variables from Strings to Values
-        if let Err(e) = convert_env_values(engine_state, stack) {
-            report_shell_error(engine_state, &e);
-        }
-
-        perf!(
-            "translate env vars after default_env.nu",
-            start_time,
-            use_color
-        );
     } else {
         eval_default_config(engine_state, stack, get_default_config(), is_env_config);
     };