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/logger_tests/test_basic_commands.nu

@@ -1,3 +1,4 @@
+use std/testing *
 use std/assert
 
 def run [
@@ -41,57 +42,57 @@ def "assert message short" [
     assert str contains $output "test message"
 }
 
-#[test]
+@test
 def critical [] {
     assert no message 99 critical
     assert message CRITICAL critical CRT
 }
 
-#[test]
+@test
 def critical_short [] {
     assert message short CRITICAL critical C
 }
 
-#[test]
+@test
 def error [] {
     assert no message CRITICAL error
     assert message ERROR error ERR
 }
 
-#[test]
+@test
 def error_short [] {
     assert message short ERROR error E
 }
 
-#[test]
+@test
 def warning [] {
     assert no message ERROR warning
     assert message WARNING warning WRN
 }
 
-#[test]
+@test
 def warning_short [] {
     assert message short WARNING warning W
 }
 
-#[test]
+@test
 def info [] {
     assert no message WARNING info
     assert message INFO info "INF" # INF has to be quoted, otherwise it is the `inf` float
 }
 
-#[test]
+@test
 def info_short [] {
     assert message short INFO info I
 }
 
-#[test]
+@test
 def debug [] {
     assert no message INFO debug
     assert message DEBUG debug DBG
 }
 
-#[test]
+@test
 def debug_short [] {
     assert message short DEBUG debug D
 }