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/dt/mod.nu

@@ -89,23 +89,23 @@ def borrow-second [from: record, current: record] {
 }
 
 # Subtract later from earlier datetime and return the unit differences as a record
-# Example:
-# > dt datetime-diff 2023-05-07T04:08:45+12:00 2019-05-10T09:59:12-07:00
-# ╭─────────────┬────╮
-# │ year        │ 3  │
-# │ month       │ 11 │
-# │ day         │ 26 │
-# │ hour        │ 23 │
-# │ minute      │ 9  │
-# │ second      │ 33 │
-# │ millisecond │ 0  │
-# │ microsecond │ 0  │
-# │ nanosecond  │ 0  │
-# ╰─────────────┴────╯
+@example "Get the difference between two dates" {
+    dt datetime-diff 2023-05-07T04:08:45+12:00 2019-05-10T09:59:12-07:00
+} --result {
+    year: 3,
+    month: 11,
+    day: 26,
+    hour: 23,
+    minute: 9,
+    second: 33,
+    millisecond: 0,
+    microsecond: 0,
+    nanosecond: 0,
+}
 export def datetime-diff [
-        later: datetime, # a later datetime
-        earlier: datetime  # earlier (starting) datetime
-    ] {
+    later: datetime, # a later datetime
+    earlier: datetime  # earlier (starting) datetime
+]: [nothing -> record] {
     if $earlier > $later {
         let start = (metadata $later).span.start
         let end = (metadata $earlier).span.end
@@ -162,10 +162,10 @@ export def datetime-diff [
 }
 
 # Convert record from datetime-diff into humanized string
-# Example:
-# > dt pretty-print-duration (dt datetime-diff 2023-05-07T04:08:45+12:00 2019-05-10T09:59:12+12:00)
-# 3yrs 11months 27days 18hrs 9mins 33secs
-export def pretty-print-duration [dur: record] {
+@example "Format the difference between two dates into a human readable string" {
+    dt pretty-print-duration (dt datetime-diff 2023-05-07T04:08:45+12:00 2019-05-10T09:59:12+12:00)
+} --result "3yrs 11months 27days 18hrs 9mins 33secs"
+export def pretty-print-duration [dur: record]: [nothing -> string] {
     mut result = ""
     if $dur.year != 0 {
         if $dur.year > 1 {