HighlightJS: Improve performance when rendering files with many lines

As part of highlighting source code using HighlightJS I noticed slow performance when using Highlight.js to highlight source code with many lines.

• example (:highlight_js FF enabled)

We need to come up with ways to improve the rendering performance.

Initial ideas

• Slice big files into smaller chunks and only highlight them once they come into view (IntersectionObserver)
• Make use of content-visibility: auto; to ensure the browser only renders what is visible to the user (limited browser support)