This changes the terminology from "focused entry" to "active entry", and
adds 4 properties that rebuild_contents should maintain for the active
entry:

- We should always have an active_entry after rebuild_contents
- The active entry's workspace should always be == to the active
multiworkspace
- If there's a thread, the active entry should always reflect that
thread
- There should always be exactly 1 active entry

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] 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)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A