Fix source code highlighting when using relative URL

What does this MR do and why?

The source code is not highlighted when viewing source code on an instance configured with a relative URL. This is because the highlight.js language definition file was not being loaded from the correct location.

This MR updates the worker-loader to resolve to the correct public path

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Before	After

How to set up and validate locally

• Configure GDK with a relative URL
  • Add relative_url_root: "/relative" to /gdk.yml
• Reconfigure GDK with gdk reconfigure
• Restart GDK with gdk restart
• Open a Source code file via the repository files browser
• The syntax should be highlighted as expected

Related to #437084 (closed)