Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] ~~Unsafe blocks (if any) have justifying comments~~
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] ~~Performance impact has been considered and is acceptable~~

Release Notes:

- Added shell completions generation for CLI with `zed --completions
<SHELL>`

Co-authored-by: silvanshade <silvanshade@users.noreply.github.com>
Co-authored-by: Yara 🏳️‍⚧️ <git@yara.blue>