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/ast/expression.rs

@@ -104,6 +104,7 @@ impl Expression {
 
     pub fn has_in_variable(&self, working_set: &StateWorkingSet) -> bool {
         match &self.expr {
+            Expr::AttributeBlock(ab) => ab.item.has_in_variable(working_set),
             Expr::BinaryOp(left, _, right) => {
                 left.has_in_variable(working_set) || right.has_in_variable(working_set)
             }
@@ -280,6 +281,7 @@ impl Expression {
             self.span = new_span;
         }
         match &mut self.expr {
+            Expr::AttributeBlock(ab) => ab.item.replace_span(working_set, replaced, new_span),
             Expr::BinaryOp(left, _, right) => {
                 left.replace_span(working_set, replaced, new_span);
                 right.replace_span(working_set, replaced, new_span);
@@ -428,6 +430,7 @@ impl Expression {
 
     pub fn replace_in_variable(&mut self, working_set: &mut StateWorkingSet, new_var_id: VarId) {
         match &mut self.expr {
+            Expr::AttributeBlock(ab) => ab.item.replace_in_variable(working_set, new_var_id),
             Expr::Bool(_) => {}
             Expr::Int(_) => {}
             Expr::Float(_) => {}