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/outline/src/outline.rs

@@ -4,6 +4,7 @@ use std::{
     sync::Arc,
 };
 
+use editor::RowHighlightOptions;
 use editor::{Anchor, AnchorRangeExt, Editor, scroll::Autoscroll};
 use fuzzy::StringMatch;
 use gpui::{
@@ -171,7 +172,10 @@ impl OutlineViewDelegate {
                 active_editor.highlight_rows::<OutlineRowHighlights>(
                     outline_item.range.start..outline_item.range.end,
                     cx.theme().colors().editor_highlighted_line_background,
-                    true,
+                    RowHighlightOptions {
+                        autoscroll: true,
+                        ..Default::default()
+                    },
                     cx,
                 );
                 active_editor.request_autoscroll(Autoscroll::center(), cx);