Add list mode to file browser in diffs

What does this MR do?

Adds toggle buttons to switch between file & tree list rendering. Tree list rendering renders the files within folders. File list rendering only renders the files paths, which gets truncated at the start of the path.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51859

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides