Use const NU_LIB_DIRS in startup (#14549)

# Description

A slower, gentler alternative to #14531, in that we're just moving one
setting *out* of `default_env.nu` in this PR ;-).

All this does is transition from using `$env.NU_LIB_DIRS` in the startup
config to `const $NU_LIB_DIRS`. Also updates the `sample_env.nu` to
reflect the changes.

Details:

Before: `$env.NU_LIB_DIRS` was unnecessary set both in `main()` and in
`default_env.nu`
After: `$env.NU_LIB_DIRS` is only set in `main()`

Before: `$env.NU_LIB_DIRS` was set to `config-dir/scripts` and
`data-dir/completions`
After: `$env.NU_LIB_DIRS` is set to an empty list, and `const
NU_LIB_DIRS` is set to the directories above

Before: Using `--include-path (-I)` would set the `$env.NU_LIB_DIRS`
After: Using `--include-path (-I)` sets the constant `$NU_LIB_DIRS`

# User-Facing Changes

There shouldn't be any breaking changes here. The `$env.NU_LIBS_DIRS`
still works for most cases. There are a few areas we need to clean-up to
make sure that the const is usable (`nu-check`, et. al.) but they will
still work in the meantime with the older `$env` version.

# Tests + Formatting

* Changed the Type-check on the `$env` version.
* Added a type check for the const version.

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

# After Submitting

Doc updates

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

@@ -47,11 +47,6 @@ $env.ENV_CONVERSIONS = {
     }
 }
 
-$env.NU_LIB_DIRS = $env.NU_LIB_DIRS? | default [
-    ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
-    ($nu.data-dir | path join 'completions') # default home for nushell completions
-]
-
 $env.NU_PLUGIN_DIRS = $env.NU_PLUGIN_DIRS | default [
     ($nu.default-config-dir | path join 'plugins') # add <nushell-config-dir>/plugins
 ]

crates/nu-utils/src/default_files/sample_env.nu

@@ -99,17 +99,17 @@ $env.ENV_CONVERSIONS = $env.ENV_CONVERSIONS | merge {
 
 # NU_LIB_DIRS
 # -----------
-# Directories in this environment variable are searched by the
+# Directories in this constant are searched by the
 # `use` and `source` commands.
 #
 # By default, the `scripts` subdirectory of the default configuration
 # directory is included:
-$env.NU_LIB_DIRS = [
+const NU_LIB_DIRS = [
     ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
     ($nu.data-dir | path join 'completions') # default home for nushell completions
 ]
-# You can replace (override) or append to this list:
-$env.NU_LIB_DIRS ++= ($nu.default-config-dir | path join 'modules')
+# You can replace (override) or append to this list by shadowing the constant
+const NU_LIB_DIRS = $NU_LIB_DIRS ++ [($nu.default-config-dir | path join 'modules')]
 
 # NU_PLUGIN_DIRS
 # --------------