Refine styling of merge conflicts (#31012)

- Improved colors
- Blank out diff hunk gutter highlights in conflict regions
- Paint conflict marker highlights all the way to the gutter

Release Notes:

- Improved the highlighting of merge conflict markers in editors.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent/src/inline_assistant.rs

@@ -1331,7 +1331,7 @@ impl InlineAssistant {
                 editor.clear_gutter_highlights::<GutterPendingRange>(cx);
             } else {
                 editor.highlight_gutter::<GutterPendingRange>(
-                    &gutter_pending_ranges,
+                    gutter_pending_ranges,
                     |cx| cx.theme().status().info_background,
                     cx,
                 )
@@ -1342,7 +1342,7 @@ impl InlineAssistant {
                 editor.clear_gutter_highlights::<GutterTransformedRange>(cx);
             } else {
                 editor.highlight_gutter::<GutterTransformedRange>(
-                    &gutter_transformed_ranges,
+                    gutter_transformed_ranges,
                     |cx| cx.theme().status().info,
                     cx,
                 )