Implement staging and unstaging hunks (#24606)

- [x] Staging hunks
- [x] Unstaging hunks
- [x] Write a randomized test
- [x] Get test passing
- [x] Fix existing bug in diff_base_byte_range computation
- [x] Remote project support
- [ ] ~~Improve performance of
buffer_range_to_unchanged_diff_base_range~~
- [ ] ~~Bug: project diff editor scrolls to top when staging/unstaging
hunk~~ existing issue
- [ ] ~~UI~~ deferred
- [x] Tricky cases
  - [x] Correctly handle acting on multiple hunks for a single file
- [x] Remove path from index when unstaging the last staged hunk, if
it's absent from HEAD, or staging the only hunk, if it's deleted in the
working copy

Release Notes:

- Add `ToggleStagedSelectedDiffHunks` action for staging and unstaging
individual diff hunks

crates/proto/src/proto.rs

@@ -440,6 +440,7 @@ messages!(
     (SyncExtensionsResponse, Background),
     (InstallExtension, Background),
     (RegisterBufferWithLanguageServers, Background),
+    (SetIndexText, Background),
 );
 
 request_messages!(
@@ -573,6 +574,7 @@ request_messages!(
     (SyncExtensions, SyncExtensionsResponse),
     (InstallExtension, Ack),
     (RegisterBufferWithLanguageServers, Ack),
+    (SetIndexText, Ack),
 );
 
 entity_messages!(
@@ -665,6 +667,7 @@ entity_messages!(
     GetPathMetadata,
     CancelLanguageServerWork,
     RegisterBufferWithLanguageServers,
+    SetIndexText,
 );
 
 entity_messages!(