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-cmd-plugin/src/util.rs

@@ -135,18 +135,24 @@ pub(crate) fn get_plugin_dirs(
     engine_state: &EngineState,
     stack: &Stack,
 ) -> impl Iterator<Item = String> {
-    // Get the NU_PLUGIN_DIRS constant or env var
+    // Get the NU_PLUGIN_DIRS from the constant and/or env var
     let working_set = StateWorkingSet::new(engine_state);
-    let value = working_set
+    let dirs_from_const = working_set
         .find_variable(b"$NU_PLUGIN_DIRS")
         .and_then(|var_id| working_set.get_constant(var_id).ok())
-        .or_else(|| stack.get_env_var(engine_state, "NU_PLUGIN_DIRS"))
-        .cloned(); // TODO: avoid this clone
-
-    // Get all of the strings in the list, if possible
-    value
+        .cloned() // TODO: avoid this clone
         .into_iter()
         .flat_map(|value| value.into_list().ok())
         .flatten()
-        .flat_map(|list_item| list_item.coerce_into_string().ok())
+        .flat_map(|list_item| list_item.coerce_into_string().ok());
+
+    let dirs_from_env = stack
+        .get_env_var(engine_state, "NU_PLUGIN_DIRS")
+        .cloned() // TODO: avoid this clone
+        .into_iter()
+        .flat_map(|value| value.into_list().ok())
+        .flatten()
+        .flat_map(|list_item| list_item.coerce_into_string().ok());
+
+    dirs_from_const.chain(dirs_from_env)
 }