Allow both NU_PLUGIN_DIRS const and env at the same time (#14553)

# Description

Fix #14544 and is also the reciprocal of #14549.

Before: If both a const and env `NU_PLUGIN_DIRS` were defined at the
same time, the env paths would not be used.
After: The directories from `const NU_PLUGIN_DIRS` are searched for a
matching filename, and if not found, `$env.NU_PLUGIN_DIRS` directories
will be searched.

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

Before: `$env.NU_PLUGIN_DIRS` was set to `plugins` in the config
directory
After: `$env.NU_PLUGIN_DIRS` is set to an empty list and `const
NU_PLUGIN_DIRS` is set to the directory above.

Also updates `sample_env.nu` to use the `const`

# User-Facing Changes

Most scenarios should work just fine as there continues to be an
`$env.NU_PLUGIN_DIRS` to append to or override.

However, there is a small chance of a breaking change if someone was
*querying* the old default `$env.NU_PLUGIN_DIRS`.

# Tests + Formatting

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

Also updated the `env` tests and added one for the `const`.

# After Submitting

Config doc updates

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

@@ -46,7 +46,3 @@ $env.ENV_CONVERSIONS = {
         to_string: { |v| $v | path expand --no-symlink | str join (char esep) }
     }
 }
-
-$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

@@ -113,13 +113,15 @@ const NU_LIB_DIRS = $NU_LIB_DIRS ++ [($nu.default-config-dir | path join 'module
 
 # NU_PLUGIN_DIRS
 # --------------
-# Directories to search for plugin binaries when calling register.
+# Directories to search for plugin binaries when calling add.
 
 # By default, the `plugins` subdirectory of the default configuration
 # directory is included:
-$env.NU_PLUGIN_DIRS = [
+const NU_PLUGIN_DIRS = [
     ($nu.default-config-dir | path join 'plugins') # add <nushell-config-dir>/plugins
 ]
+# You can replace (override) or append to this list by shadowing the constant
+const NU_PLUGIN_DIRS = $NU_PLUGIN_DIRS ++ [($nu.default-config-dir | path join 'plugins')]
 
 # Appending to the OS path is a common configuration task.
 # Because of the previous ENV_CONVERSIONS (performed internally