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/editor/src/items.rs

@@ -288,7 +288,7 @@ impl FollowableItem for Editor {
                     }
                     true
                 }
-                EditorEvent::ExcerptsRemoved { ids } => {
+                EditorEvent::ExcerptsRemoved { ids, .. } => {
                     update
                         .deleted_excerpts
                         .extend(ids.iter().map(ExcerptId::to_proto));