On macOS, traditionally when a keyboard shortcut is activated, the menu
in the menu bar flashes to indicate that the action was recognised.

<img width="289" height="172" alt="image"
src="https://github.com/user-attachments/assets/a03ecd2f-f159-4f82-b4fd-227f34393703"
/>

This PR adds this functionality to GPUI, where when a keybind is pressed
that triggers an action in the menu, the menu flashes.

Release Notes:

- N/A