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-std/testing.nu

@@ -14,43 +14,29 @@ def "nu-complete threads" [] {
 # test and test-skip annotations may be used multiple times throughout the module as the function names are stored in a list
 # Other annotations should only be used once within a module file
 # If you find yourself in need of multiple before- or after- functions it's a sign your test suite probably needs redesign
-def valid-annotations [] {
-    {
-        "#[test]": "test",
-        "#[ignore]": "test-skip",
-        "#[before-each]": "before-each"
-        "#[before-all]": "before-all"
-        "#[after-each]": "after-each"
-        "#[after-all]": "after-all"
-    }
+const valid_annotations = {
+    "test": "test",
+    "ignore": "test-skip",
+    "before-each": "before-each"
+    "before-all": "before-all"
+    "after-each": "after-each"
+    "after-all": "after-all"
 }
 
 # Returns a table containing the list of function names together with their annotations (comments above the declaration)
 def get-annotated [
     file: path
 ]: nothing -> table<function_name: string, annotation: string> {
-    let raw_file = (
-        open $file
-        | lines
-        | enumerate
-        | flatten
-    )
-
-    $raw_file
-    | where item starts-with def and index > 0
-    | insert annotation {|x|
-        $raw_file
-        | get ($x.index - 1)
-        | get item
-        | str trim
-    }
-    | where annotation in (valid-annotations|columns)
-    | reject index
-    | update item {
-        split column --collapse-empty ' '
-        | get column2.0
-    }
-    | rename function_name
+    ^$nu.current-exe -c $'
+        source `($file)`
+        scope commands
+        | select name attributes
+        | where attributes != []
+        | to nuon
+    '
+    | from nuon
+    | update attributes { get name | each {|x| $valid_annotations | get -i $x } | first }
+    | rename function_name annotation
 }
 
 # Takes table of function names and their annotations such as the one returned by get-annotated
@@ -72,10 +58,6 @@ def create-test-record []: nothing -> record<before-each: string, after-each: st
 
     let test_record = (
         $input
-        | update annotation {|x|
-            valid-annotations
-            | get $x.annotation
-        }
         | group-by --to-table annotation
         | update items {|x|
             $x.items.function_name