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/git_ui/src/git_ui.rs

@@ -3,6 +3,7 @@ use std::any::Any;
 use ::settings::Settings;
 use command_palette_hooks::CommandPaletteFilter;
 use commit_modal::CommitModal;
+use editor::Editor;
 mod blame_ui;
 use git::{
     repository::{Branch, Upstream, UpstreamTracking, UpstreamTrackingStatus},
@@ -20,6 +21,7 @@ pub mod branch_picker;
 mod commit_modal;
 pub mod commit_tooltip;
 mod commit_view;
+mod conflict_view;
 pub mod git_panel;
 mod git_panel_settings;
 pub mod onboarding;
@@ -35,6 +37,11 @@ pub fn init(cx: &mut App) {
 
     editor::set_blame_renderer(blame_ui::GitBlameRenderer, cx);
 
+    cx.observe_new(|editor: &mut Editor, _, cx| {
+        conflict_view::register_editor(editor, editor.buffer().clone(), cx);
+    })
+    .detach();
+
     cx.observe_new(|workspace: &mut Workspace, _, cx| {
         ProjectDiff::register(workspace, cx);
         CommitModal::register(workspace);