File-by-file merge request diff navigation POC

What does this MR do?

This is a frontend only proof of concept to show diffs one file at a time instead of all shown at once. This functionality is hidden behind two feature flags.

• file_by_file_default shows all diffs one file at a time.
• file_by_file_option enables a flag under "Show whitespace changes" to toggle file by file off and on.

Current multi-file display

Areas to be changed highlighted in red and listed below:

• Remove file list toggle
• Highlight selected file
• Add toggle file by file checkbox
• Only show selected file

Proposed file by file display

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides

Performance and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Closes #62536 (closed)