Add syntax highlighting for Gleam (repository > view file)

Proposal

Please add support for syntax highlighting of the Gleam programming language

Details for implementation

• Syntax highlighting development guidelines (repository blob viewer)
• syntax highlighting in blobs is leveraging highlight.js — seems like potentially it's a matter of leveraging https://github.com/gleam-lang/gleam-highlight.js/
• for diffs (commits, MRs, etc.) we leverage Rouge gem — so it's a matter of adding support for Gleam there.