This adds a new version of the project diff editor to go alongside the
new git panel.
The basics seem to be working, but still todo:
* [ ] Fix untracked files
* [ ] Fix deleted files
* [ ] Show commit message editor at top
* [x] Handle empty state
* [x] Fix panic where locator sometimes seeks to wrong excerpt
Release Notes:
- N/A
Hopefully this makes it a bit easier for new contributors to dive into
the codebase :)
Release Notes:
- Improved documentation for many core editor types
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Co-authored-by: Nathan Sobo <nathan@zed.dev>