Implement fallback to Rouge

What does this MR do and why?

Implements a fallback to Rouge when a language is not supported by Highlight.js and adds tracking events along the way.

⚠ changes in this MR are behind the highlight_js feature flag.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the highlight_js feature flag
2. View a file from the repository file browser

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #365442 (closed)