Add a new Rapid Diffs view for Merge Requests

Context

This is an initial step to unlock development of Rapid Diffs.

The aim of this view is to provide a way to instantiate and render diffs in a completely new way.

Details

• Work behind the feature flag defined in Add rapid_diffs feature flag for Rapid Diffs ef... (#450994 - closed)
• To be determined: Access:
  • which route should we use? See thread below for discussion.
  • how does a user toggle between old and new views?

Out of scope for this issue

• Don't: Bootstrap the Rapid Diffs frontend app or any other frontend content. - just creating the route/view under the feature flag and rendering an empty page to start. Anything else is extra