Highlight merge request conflicts displayed in diff

Within #232484 (closed) we've displayed merge request conflicts in diff. No frontend work has been performed yet, so now it looks like:

Problem to solve

Even though it uses Git's nomenclature, it's not very clear from the diff which changes are our and which already exist on the target branch

Proposal

1. Highlight the conflicts blocks properly, probably the same way as on our Resolve conflicts page.
   1. Should support syntax highlighting (which our conflict editor does).
2. Use different labels for marking conflicts instead of filename (lib/wit.ex on the screenshot example) - that would help a user to identify ours/theirs changes.

Feature flag

• name: display_merge_conflicts_in_diff
• rollout_issue: #276918 (closed)

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.