Fix source code scrolling

What does this MR do and why?

Fixes the issue where line numbers were not sticky while scrolling

Note, changes in this MR are behind the highlight_js_worker feature flag.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run echo "Feature.enable(:highlight_js_worker)" | gdk rails c
2. Open a source code blob via the repository files browser (sample.json)
3. While scrolling on the x-axis the line numbers should stay visible.

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 #415753 (closed)