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/tests/test_dirs.nu

@@ -1,3 +1,4 @@
+use std/testing *
 use std/assert
 use std/log
 
@@ -5,7 +6,7 @@ use std/log
 # Each 'use' for that module in the test script will execute the def --env block.
 # PWD at the time of the `use` will be what the export def --env block will see.
 
-#[before-each]
+@before-each
 def before-each [] {
     # need some directories to play with
     let base_path = ($nu.temp-path | path join $"test_dirs_(random uuid)")
@@ -17,7 +18,7 @@ def before-each [] {
     {base_path: $base_path, path_a: $path_a, path_b: $path_b}
 }
 
-#[after-each]
+@after-each
 def after-each [] {
     let base_path = $in.base_path
     cd $base_path
@@ -36,7 +37,7 @@ def cur_ring_check [expect_dir:string, expect_position: int scenario:string] {
     assert equal $expect_position $env.DIRS_POSITION $"position in ring after ($scenario)"
 }
 
-#[test]
+@test
 def dirs_command [] {
     # careful with order of these statements!
     # must capture value of $in before executing `use`s
@@ -87,7 +88,7 @@ def dirs_command [] {
     assert equal $env.PWD $c.base_path "drop changes PWD (regression test for #9449)"
 }
 
-#[test]
+@test
 def dirs_next [] {
     # must capture value of $in before executing `use`s
     let $c = $in
@@ -109,7 +110,7 @@ def dirs_next [] {
 
 }
 
-#[test]
+@test
 def dirs_cd [] {
     # must capture value of $in before executing `use`s
     let $c = $in
@@ -134,7 +135,7 @@ def dirs_cd [] {
     assert equal [$c.path_b $c.path_b] $env.DIRS_LIST "cd updated both positions in ring"
 }
 
-#[test]
+@test
 def dirs_goto_bug10696 [] {
     let $c = $in
     cd $c.base_path
@@ -148,7 +149,7 @@ def dirs_goto_bug10696 [] {
     assert equal $env.PWD $c.path_b "goto other, then goto to come back returns to same directory"
 }
 
-#[test]
+@test
 def dirs_goto [] {
     let $c = $in
     cd $c.base_path