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.

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

assets/themes/one/one.json

@@ -99,6 +99,8 @@
         "version_control.added": "#27a657ff",
         "version_control.modified": "#d3b020ff",
         "version_control.deleted": "#e06c76ff",
+        "version_control.conflict_marker.ours": "#a1c1811a",
+        "version_control.conflict_marker.theirs": "#74ade81a",
         "conflict": "#dec184ff",
         "conflict.background": "#dec1841a",
         "conflict.border": "#5d4c2fff",