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/multi_buffer/src/multi_buffer_tests.rs

@@ -635,7 +635,7 @@ fn test_excerpt_events(cx: &mut App) {
                     predecessor,
                     excerpts,
                 } => follower.insert_excerpts_with_ids_after(predecessor, buffer, excerpts, cx),
-                Event::ExcerptsRemoved { ids } => follower.remove_excerpts(ids, cx),
+                Event::ExcerptsRemoved { ids, .. } => follower.remove_excerpts(ids, cx),
                 Event::Edited { .. } => {
                     *follower_edit_event_count.write() += 1;
                 }