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_xml.nu

@@ -1,7 +1,8 @@
+use std/testing *
 use std/xml *
 use std/assert
 
-#[before-each]
+@before-each
 def before-each [] {
     {sample_xml: ('
         <a>
@@ -17,7 +18,7 @@ def before-each [] {
     }
 }
 
-#[test]
+@test
 def xml_xaccess [] {
     let sample_xml = $in.sample_xml
 
@@ -28,7 +29,7 @@ def xml_xaccess [] {
     assert equal ($sample_xml | xaccess [* * * {|e| $e.attributes != {}}]) [[tag, attributes, content]; [c, {a: b}, []]]
 }
 
-#[test]
+@test
 def xml_xupdate [] {
     let sample_xml = $in.sample_xml
 
@@ -37,7 +38,7 @@ def xml_xupdate [] {
     assert equal ($sample_xml | xupdate [* * * {|e| $e.attributes != {}}] {|x| $x | update content ['xml']}) {tag: a, attributes: {}, content: [[tag, attributes, content]; [b, {}, [[tag, attributes, content]; [c, {a: b}, [xml]]]], [c, {}, []], [d, {}, [[tag, attributes, content]; [e, {}, [[tag, attributes, content]; [null, null, z]]], [e, {}, [[tag, attributes, content]; [null, null, x]]]]]]}
 }
 
-#[test]
+@test
 def xml_xinsert [] {
     let sample_xml = $in.sample_xml