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/project/src/project.rs

@@ -29,7 +29,10 @@ pub mod search_history;
 mod yarn;
 
 use crate::git_store::GitStore;
-pub use git_store::git_traversal::{ChildEntriesGitIter, GitEntry, GitEntryRef, GitTraversal};
+pub use git_store::{
+    ConflictRegion, ConflictSet, ConflictSetSnapshot, ConflictSetUpdate,
+    git_traversal::{ChildEntriesGitIter, GitEntry, GitEntryRef, GitTraversal},
+};
 
 use anyhow::{Context as _, Result, anyhow};
 use buffer_store::{BufferStore, BufferStoreEvent};