project_panel: Fix worktree root rename

[0xtimsb]

Closes #7923

This PR fixes root worktree renaming by:

1. Handling the case where new_path is the new root name instead of a relative path from the root.
2. #20313 added functionality to watch for root worktree renames made externally, e.g., via Finder. This PR avoids relying on that watcher because, when renaming explicitly from Zed, we can eagerly perform the necessary work (of course after fs rename) instead of waiting for the watcher to detect the rename. This prevents UI glitches during renaming root.

Todo:

• Fix wrong abs paths when root is renamed
• Fix explicit scan entry func to handle renamed root dir
• Tests
• Test on Linux
• Test on Windows (Root-level rename currently fails on Windows, with error: "cannot rename directory in use"

Release Notes:

• Fixed an issue where worktree root name couldn't be renamed in project panel.