The git worktree picker shows worktrees open in the current window, but
had no way to remove them. This adds a "Remove Worktree from Window"
button (same `X` icon as the recent projects picker's "Remove Project
from Window") that closes the corresponding workspace in the
multi-workspace, without deleting the git worktree itself.

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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Added a button to the git worktree picker to remove an open worktree
from the current window.