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

src/main.rs

@@ -25,7 +25,7 @@ use nu_lsp::LanguageServer;
 use nu_path::canonicalize_with;
 use nu_protocol::{
     engine::EngineState, report_shell_error, ByteStream, Config, IntoValue, PipelineData,
-    ShellError, Span, Spanned, Value,
+    ShellError, Span, Spanned, Type, Value,
 };
 use nu_std::load_standard_library;
 use nu_utils::perf;
@@ -147,13 +147,25 @@ fn main() -> Result<()> {
 
     let mut default_nu_lib_dirs_path = nushell_config_path.clone();
     default_nu_lib_dirs_path.push("scripts");
-    engine_state.add_env_var(
-        "NU_LIB_DIRS".to_string(),
+    // env.NU_LIB_DIRS to be replaced by constant (below) - Eventual deprecation
+    // but an empty list for now to allow older code to work
+    engine_state.add_env_var("NU_LIB_DIRS".to_string(), Value::test_list(vec![]));
+
+    let mut working_set = nu_protocol::engine::StateWorkingSet::new(&engine_state);
+    let var_id = working_set.add_variable(
+        b"$NU_LIB_DIRS".into(),
+        Span::unknown(),
+        Type::List(Box::new(Type::String)),
+        false,
+    );
+    working_set.set_variable_const_val(
+        var_id,
         Value::test_list(vec![
             Value::test_string(default_nu_lib_dirs_path.to_string_lossy()),
             Value::test_string(default_nushell_completions_path.to_string_lossy()),
         ]),
     );
+    engine_state.merge_delta(working_set.render())?;
 
     let mut default_nu_plugin_dirs_path = nushell_config_path;
     default_nu_plugin_dirs_path.push("plugins");
@@ -273,7 +285,15 @@ fn main() -> Result<()> {
             .map(|x| Value::string(x.trim().to_string(), span))
             .collect();
 
-        engine_state.add_env_var("NU_LIB_DIRS".into(), Value::list(vals, span));
+        let mut working_set = nu_protocol::engine::StateWorkingSet::new(&engine_state);
+        let var_id = working_set.add_variable(
+            b"$NU_LIB_DIRS".into(),
+            span,
+            Type::List(Box::new(Type::String)),
+            false,
+        );
+        working_set.set_variable_const_val(var_id, Value::list(vals, span));
+        engine_state.merge_delta(working_set.render())?;
     }
     perf!("NU_LIB_DIRS setup", start_time, use_color);