This PR allows users to create a new thread based off a git worktree
that already exists or has a custom name. User's can also choose what
branch they want the newly generated worktree to be based off of.

The UI still needs some polish, but I'm merging this early to get the
team using this before our preview launch. I'll be active today and
tomorrow before launch to fix any nits we have with the UI.

Functionality of this feature works! And I have a basic test to prevent
regressions


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 or Added/Fixed/Improved ...

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Co-authored-by: cameron <cameron.studdstreet@gmail.com>