Resolve "Only load syntax highlighting css when selected"

What does this MR do?

Shaves 5% off application.css filesize, and pass the savings onto yooooooou.

Instead of loading every syntax highlighting CSS, only load the selected highlighting theme.

What are the relevant issue numbers?

Closes #56873 (closed) Fixes #57739 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer