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.

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

crates/git/src/repository.rs

@@ -133,7 +133,7 @@ pub struct CommitSummary {
     pub has_parent: bool,
 }
 
-#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+#[derive(Clone, Debug, Default, Hash, PartialEq, Eq)]
 pub struct CommitDetails {
     pub sha: SharedString,
     pub message: SharedString,