Implement the "Kebab"/Ellipsis menu for Rapid Diffs Diff File Header

Adding the 3-dots menu to the Rapid Diffs header wasn't originally scoped in the issue and was added after a review.

To mitigate scope creep and avoid creating an epic-sized issue, we should implement the ellipsis menu for Diff Files in Rapid Diffs in this issue:

• Kebab menu
• View file at SHA
• Edit file in single file editor