Highlight merge conflicts and provide for resolving them (#28065)

TODO:

- [x] Make it work in the project diff:
  - [x] Support non-singleton buffers
  - [x] Adjust excerpt boundaries to show full conflicts
- [x] Write tests for conflict-related events and state management
- [x] Prevent hunk buttons from appearing inside conflicts
- [x] Make sure it works over SSH, collab
- [x] Allow separate theming of markers

Bonus:

- [ ] Count of conflicts in toolbar
- [ ] Keyboard-driven navigation and resolution
- [ ] ~~Inlay hints to contextualize "ours"/"theirs"~~

Release Notes:

- Implemented initial support for resolving merge conflicts.

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/outline_panel/src/outline_panel.rs

@@ -5028,7 +5028,7 @@ fn subscribe_for_editor_events(
                         .extend(excerpts.iter().map(|&(excerpt_id, _)| excerpt_id));
                     outline_panel.update_fs_entries(editor.clone(), debounce, window, cx);
                 }
-                EditorEvent::ExcerptsRemoved { ids } => {
+                EditorEvent::ExcerptsRemoved { ids, .. } => {
                     let mut ids = ids.iter().collect::<HashSet<_>>();
                     for excerpts in outline_panel.excerpts.values_mut() {
                         excerpts.retain(|excerpt_id, _| !ids.remove(excerpt_id));