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/std/bench/mod.nu

@@ -1,45 +1,34 @@
 # run a piece of `nushell` code multiple times and measure the time of execution.
 #
 # this command returns a benchmark report of the following form:
-# ```
-# record<
-#   mean: duration
-#   std: duration
-#   times: list<duration>
-# >
-# ```
 #
 # > **Note**
 # > `std bench --pretty` will return a `string`.
-#
-# # Examples
-#     measure the performance of simple addition
-#     > std bench { 1 + 2 } -n 10 | table -e
-#     ╭───────┬────────────────────╮
-#     │ mean  │ 4µs 956ns          │
-#     │ std   │ 4µs 831ns          │
-#     │       │ ╭───┬────────────╮ │
-#     │ times │ │ 0 │ 19µs 402ns │ │
-#     │       │ │ 1 │  4µs 322ns │ │
-#     │       │ │ 2 │  3µs 352ns │ │
-#     │       │ │ 3 │  2µs 966ns │ │
-#     │       │ │ 4 │        3µs │ │
-#     │       │ │ 5 │   3µs 86ns │ │
-#     │       │ │ 6 │   3µs 84ns │ │
-#     │       │ │ 7 │  3µs 604ns │ │
-#     │       │ │ 8 │   3µs 98ns │ │
-#     │       │ │ 9 │  3µs 653ns │ │
-#     │       │ ╰───┴────────────╯ │
-#     ╰───────┴────────────────────╯
-#
-#     get a pretty benchmark report
-#     > std bench { 1 + 2 } --pretty
-#     3µs 125ns +/- 2µs 408ns
+@example "measure the performance of simple addition" { bench { 1 + 2 } -n 10 } --result {
+    mean: (4µs + 956ns)
+    std: (4µs + 831ns)
+    times: [
+        (19µs + 402ns)
+        ( 4µs + 322ns)
+        ( 3µs + 352ns)
+        ( 2µs + 966ns)
+        ( 3µs        )
+        ( 3µs +  86ns)
+        ( 3µs +  84ns)
+        ( 3µs + 604ns)
+        ( 3µs +  98ns)
+        ( 3µs + 653ns)
+    ]
+}
+@example "get a pretty benchmark report" { bench { 1 + 2 } --pretty } --result "3µs 125ns +/- 2µs 408ns"
 export def main [
     code: closure  # the piece of `nushell` code to measure the performance of
     --rounds (-n): int = 50  # the number of benchmark rounds (hopefully the more rounds the less variance)
     --verbose (-v) # be more verbose (namely prints the progress)
     --pretty # shows the results in human-readable format: "<mean> +/- <stddev>"
+]: [
+    nothing -> record<mean: duration, std: duration, times: list<duration>>
+    nothing -> string
 ] {
     let times: list<duration> = (
         seq 1 $rounds | each {|i|