Add user autoload directory (#14669)

# Description

Adds a user-level (non-vendor) autoload directory:

```
($nu.default-config-dir)/autoload
```

Currently this is the only directory. We can consider adding others if
needed.

Related: As a separate PR, I'm going to try to restore the ability to
set `$env.NU_AUTOLOAD_DIRS` during startup.

# User-Facing Changes

Files in `$nu.default-config-dir/autoload` will be autoload at startup.
These files will be loaded after any vendor autoloads, so that a user
can override the vendor settings.

# Tests + Formatting

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

# After Submitting

TODO; add a `$nu.user-autoload-dirs` constant.

Doc updates

crates/nu-cli/tests/completions/mod.rs

@@ -1357,7 +1357,7 @@ fn variables_completions() {
     // Test completions for $nu
     let suggestions = completer.complete("$nu.", 4);
 
-    assert_eq!(18, suggestions.len());
+    assert_eq!(19, suggestions.len());
 
     let expected: Vec<String> = vec![
         "cache-dir".into(),
@@ -1377,6 +1377,7 @@ fn variables_completions() {
         "plugin-path".into(),
         "startup-time".into(),
         "temp-path".into(),
+        "user-autoload-dirs".into(),
         "vendor-autoload-dirs".into(),
     ];