## Summary

This PR introduces an announcement banner for Delta that will be shown
in the next Zed release.

## Self-review

- [x] I've reviewed my diff for quality, security, reliability, and
performance.
- [x] UI changes follow the
[checklist](https://zed.dev/docs/development/ui-checklist).
- [x] Tests cover the new or changed behavior.

Release Notes:

- N/A