switch: Add AccessKit role, toggle state, and label (#2642)

## Description

`Switch` rendered no accessibility annotations at all, so assistive
technology could not perceive it.

This annotates the interactive element with `Role::Switch`
(`accesskit_macos` maps it to the checkbox role with the switch
subrole), `aria_toggled`, and the label text.

## How to Test

Use accessibility tools on a `Switch` element.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] ~Passed `cargo run` for story tests related to the changes.~ **Not
needed.**
- [ ] ~Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)~ **Only tested on macOS.**