assistant2: Add `ChatNotice` component (#11262)

This PR adds a new `ChatNotice` component for displaying notices within
the assistant.

We will be able to use this for allowing the user to confirm a project
index request.

<img width="518" alt="Screenshot 2024-05-01 at 2 41 31 PM"
src="https://github.com/zed-industries/zed/assets/1486634/3beaeeaf-6046-4284-ac0a-4248e1f9ac20">

Release Notes:

- N/A