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

tests/repl/test_env.rs

@@ -17,8 +17,18 @@ fn shorthand_env_3() -> TestResult {
 }
 
 #[test]
-fn default_nu_lib_dirs_type() {
+fn default_nu_lib_dirs_env_type() {
+    // Previously, this was a list<string>
+    // While we are transitioning to const NU_LIB_DIRS
+    // the env version will be empty, and thus a
+    // list<any>
     let actual = nu!("$env.NU_LIB_DIRS | describe");
+    assert_eq!(actual.out, "list<any>");
+}
+
+#[test]
+fn default_nu_lib_dirs_type() {
+    let actual = nu!("$NU_LIB_DIRS | describe");
     assert_eq!(actual.out, "list<string>");
 }