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.

crates/nu-cli/src/completions/command_completions.rs

@@ -43,7 +43,7 @@ impl CommandCompletion {
 
         let paths = working_set.permanent_state.get_env_var_insensitive("path");
 
-        if let Some(paths) = paths {
+        if let Some((_, paths)) = paths {
             if let Ok(paths) = paths.as_list() {
                 for path in paths {
                     let path = path.coerce_str().unwrap_or_default();

crates/nu-cli/src/eval_cmds.rs

@@ -1,5 +1,5 @@
 use log::info;
-use nu_engine::{convert_env_values, eval_block};
+use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_protocol::{
     cli_error::report_compile_error,
@@ -50,9 +50,6 @@ pub fn evaluate_commands(
         }
     }
 
-    // Translate environment variables from Strings to Values
-    convert_env_values(engine_state, stack)?;
-
     // Parse the source code
     let (block, delta) = {
         if let Some(ref t_mode) = table_mode {

crates/nu-cli/src/eval_file.rs

@@ -1,6 +1,6 @@
 use crate::util::{eval_source, print_pipeline};
 use log::{info, trace};
-use nu_engine::{convert_env_values, eval_block};
+use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_path::canonicalize_with;
 use nu_protocol::{
@@ -22,9 +22,6 @@ pub fn evaluate_file(
     stack: &mut Stack,
     input: PipelineData,
 ) -> Result<(), ShellError> {
-    // Convert environment variables from Strings to Values and store them in the engine state.
-    convert_env_values(engine_state, stack)?;
-
     let cwd = engine_state.cwd_as_string(Some(stack))?;
 
     let file_path =