Add 'number' command for enumeration (#7871)

# Description

This adds a `number` command that will enumerate the input, and add an
`index` and `item` record for each item. The `index` is the number of
the item in the input stream, and `item` is the original value of the
item.

```
> ls | number | get 14
╭───────┬────────────────────────────╮
│ index │ 14                         │
│       │ ╭──────────┬─────────────╮ │
│ item  │ │ name     │ crates      │ │
│       │ │ type     │ dir         │ │
│       │ │ size     │ 832 B       │ │
│       │ │ modified │ 2 weeks ago │ │
│       │ ╰──────────┴─────────────╯ │
╰───────┴────────────────────────────╯
```

# User-Facing Changes

This adds a `number` command.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-color-config/src/style_computer.rs

@@ -226,10 +226,10 @@ fn test_computable_style_static() {
     let style2 = Style::default().underline();
     // Create a "dummy" style_computer for this test.
     let dummy_engine_state = EngineState::new();
-    let mut dummy_stack = Stack::new();
+    let dummy_stack = Stack::new();
     let style_computer = StyleComputer::new(
         &dummy_engine_state,
-        &mut dummy_stack,
+        &dummy_stack,
         HashMap::from([
             ("string".into(), ComputableStyle::Static(style1)),
             ("row_index".into(), ComputableStyle::Static(style2)),