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-protocol/src/errors/parse_error.rs

@@ -543,6 +543,13 @@ pub enum ParseError {
         help("try assigning to a variable or a cell path of a variable")
     )]
     AssignmentRequiresVar(#[label("needs to be a variable")] Span),
+
+    #[error("Attributes must be followed by a definition.")]
+    #[diagnostic(
+        code(nu::parser::attribute_requires_definition),
+        help("try following this line with a `def` or `extern` definition")
+    )]
+    AttributeRequiresDefinition(#[label("must be followed by a definition")] Span),
 }
 
 impl ParseError {
@@ -634,6 +641,7 @@ impl ParseError {
             ParseError::ExtraTokensAfterClosingDelimiter(s) => *s,
             ParseError::AssignmentRequiresVar(s) => *s,
             ParseError::AssignmentRequiresMutableVar(s) => *s,
+            ParseError::AttributeRequiresDefinition(s) => *s,
         }
     }
 }