add attr category @category to custom command attributes (#15137)

# Description

This PR adds the `@category` attribute to nushell for use with custom
commands.

### Example Code
```nushell
# Some example with category
@category "math"
@search-terms "addition"
@example "add two numbers together" {
    blah 5 6
} --result 11
def blah [
  a: int # First number to add
  b: int # Second number to add
  ] {
    $a + $b
}
```
#### Source & Help
```nushell
❯ source blah.nu
❯ help blah
Some example with category

Search terms: addition

Usage:
  > blah <a> <b>

Flags:
  -h, --help: Display the help message for this command

Parameters:
  a <int>: First number to add
  b <int>: Second number to add

Input/output types:
  ╭─#─┬─input─┬─output─╮
  │ 0 │ any   │ any    │
  ╰───┴───────┴────────╯

Examples:
  add two numbers together
  > blah 5 6
  11
```
#### Show the category
```nushell
❯ help commands | where name == blah
╭─#─┬─name─┬─category─┬─command_type─┬────────description─────────┬─────params─────┬──input_output──┬─search_terms─┬─is_const─╮
│ 0 │ blah │ math     │ custom       │ Some example with category │ [table 3 rows] │ [list 0 items] │ addition     │ false    │
╰───┴──────┴──────────┴──────────────┴────────────────────────────┴────────────────┴────────────────┴──────────────┴──────────╯
```

# User-Facing Changes
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/cc @Bahex

crates/nu-cmd-lang/src/core_commands/attr/category.rs

@@ -0,0 +1,61 @@
+use nu_engine::command_prelude::*;
+
+#[derive(Clone)]
+pub struct AttrCategory;
+
+impl Command for AttrCategory {
+    fn name(&self) -> &str {
+        "attr category"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("attr category")
+            .input_output_type(Type::Nothing, Type::list(Type::String))
+            .allow_variants_without_examples(true)
+            .required(
+                "category",
+                SyntaxShape::String,
+                "Category of the custom command.",
+            )
+            .category(Category::Core)
+    }
+
+    fn description(&self) -> &str {
+        "Attribute for adding a category to custom commands."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let arg: String = call.req(engine_state, stack, 0)?;
+        Ok(Value::string(arg, call.head).into_pipeline_data())
+    }
+
+    fn run_const(
+        &self,
+        working_set: &StateWorkingSet,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let arg: String = call.req_const(working_set, 0)?;
+        Ok(Value::string(arg, call.head).into_pipeline_data())
+    }
+
+    fn is_const(&self) -> bool {
+        true
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Add a category to a custom command",
+            example: r###"# Double numbers
+    @category math
+    def double []: [number -> number] { $in * 2 }"###,
+            result: None,
+        }]
+    }
+}

crates/nu-cmd-lang/src/core_commands/attr/mod.rs

@@ -1,5 +1,7 @@
+mod category;
 mod example;
 mod search_terms;
 
+pub use category::AttrCategory;
 pub use example::AttrExample;
 pub use search_terms::AttrSearchTerms;