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-engine/src/compile/expression.rs

@@ -72,6 +72,14 @@ pub(crate) fn compile_expression(
     };
 
     match &expr.expr {
+        Expr::AttributeBlock(ab) => compile_expression(
+            working_set,
+            builder,
+            &ab.item,
+            redirect_modes,
+            in_reg,
+            out_reg,
+        ),
         Expr::Bool(b) => lit(builder, Literal::Bool(*b)),
         Expr::Int(i) => lit(builder, Literal::Int(*i)),
         Expr::Float(f) => lit(builder, Literal::Float(*f)),

crates/nu-engine/src/scope.rs

@@ -106,12 +106,27 @@ impl<'e, 's> ScopeData<'e, 's> {
                     })
                     .collect();
 
+                let attributes = decl
+                    .attributes()
+                    .into_iter()
+                    .map(|(name, value)| {
+                        Value::record(
+                            record! {
+                                "name" => Value::string(name, span),
+                                "value" => value,
+                            },
+                            span,
+                        )
+                    })
+                    .collect();
+
                 let record = record! {
                     "name" => Value::string(String::from_utf8_lossy(command_name), span),
                     "category" => Value::string(signature.category.to_string(), span),
                     "signatures" => self.collect_signatures(&signature, span),
                     "description" => Value::string(decl.description(), span),
                     "examples" => Value::list(examples, span),
+                    "attributes" => Value::list(attributes, span),
                     "type" => Value::string(decl.command_type().to_string(), span),
                     "is_sub" => Value::bool(decl.is_sub(), span),
                     "is_const" => Value::bool(decl.is_const(), span),