Add a feature flag for syntax highlighting using HighlightJS

Merged Jacques Erasmus requested to merge 345812-highlightjs-feature-flag into master

What does this MR do and why?

This adds a feature flag for syntax highlighting using HighlightJS.

Changes in this MR are part of a bigger effort to add support for syntax highlighting using HighlightJS..

Screenshots or screen recordings

N/A

MR acceptance checklist

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

Related to #345812 (closed)