WIP: Added Diff Viewer to new VUE based MR page

What does this MR do?

Adds the generic diff viewer to the new MR page. This brings support for all non-text files (Images + Download atm). As this is then using the generic view it will automatically benefit from any future updates/fixes, etc.

I also added general tests to spec\javascripts\diffs\components\diff_content_spec.js.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#42882 (closed)