Add placeholder for extension slash commands (#12387)

This PR adds a slightly better placeholder for extension slash commands
than the current "TODO" text.

Right now we display the command name and an arbitrary icon.

<img width="644" alt="Screenshot 2024-05-28 at 12 15 07 PM"
src="https://github.com/zed-industries/zed/assets/1486634/11761797-5ccc-4209-8b00-70b714f10a78">

Release Notes:

- N/A