Fallback to highlighting HAML with Rouge

What does this MR do and why?

Highlight.js does not highlight HAML files correctly. I've opened an issue in their repo and implemented a temporary fallback as part of this MR until the upstream issue is fixed.

Screenshots or screen recordings

before	after
Highlighted with Highlight.js	Highlighted with Rouge

How to set up and validate locally

1. View a HAML file in the repository file viewer
   • sample.haml
2. The file should highlight as expected

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.