Custom command attributes (#14906)

# Description
Add custom command attributes.

- Attributes are placed before a command definition and start with a `@`
character.
- Attribute invocations consist of const command call. The command's
name must start with "attr ", but this prefix is not used in the
invocation.
- A command named `attr example` is invoked as an attribute as
`@example`
-   Several built-in attribute commands are provided as part of this PR
    -   `attr example`: Attaches an example to the commands help text
        ```nushell
        # Double numbers
        @example "double an int"  { 5 | double }   --result 10
        @example "double a float" { 0.5 | double } --result 1.0
        def double []: [number -> number] {
            $in * 2
        }
        ```
    -   `attr search-terms`: Adds search terms to a command
    -   ~`attr env`: Equivalent to using `def --env`~
- ~`attr wrapped`: Equivalent to using `def --wrapped`~ shelved for
later discussion
    -   several testing related attributes in `std/testing`
- If an attribute has no internal/special purpose, it's stored as
command metadata that can be obtained with `scope commands`.
- This allows having attributes like `@test` which can be used by test
runners.
-   Used the `@example` attribute for `std` examples.
-   Updated the std tests and test runner to use `@test` attributes
-   Added completions for attributes

# User-Facing Changes
Users can add examples to their own command definitions, and add other
arbitrary attributes.

# Tests + Formatting

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

# After Submitting
- Add documentation about the attribute syntax and built-in attributes
- `help attributes`

---------

Co-authored-by: 132ikl <132@ikl.sh>

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

@@ -1213,6 +1213,44 @@ fn flag_completions() {
     match_suggestions(&expected, &suggestions);
 }
 
+#[test]
+fn attribute_completions() {
+    // Create a new engine
+    let (_, _, engine, stack) = new_engine();
+
+    // Instantiate a new completer
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+    // Test completions for the 'ls' flags
+    let suggestions = completer.complete("@", 1);
+
+    // Only checking for the builtins and not the std attributes
+    let expected: Vec<String> = vec!["example".into(), "search-terms".into()];
+
+    // Match results
+    match_suggestions(&expected, &suggestions);
+}
+
+#[test]
+fn attributable_completions() {
+    // Create a new engine
+    let (_, _, engine, stack) = new_engine();
+
+    // Instantiate a new completer
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+    // Test completions for the 'ls' flags
+    let suggestions = completer.complete("@example; ", 10);
+
+    let expected: Vec<String> = vec![
+        "def".into(),
+        "export def".into(),
+        "export extern".into(),
+        "extern".into(),
+    ];
+
+    // Match results
+    match_suggestions(&expected, &suggestions);
+}
+
 #[test]
 fn folder_with_directorycompletions() {
     // Create a new engine