Syntax highlighting in other forges

[zkat]

#82 has landed! so let's go ahead and start adding highlighting support for other forges?

• Gitlab - this says it uses THREE different systems: https://github.com/rouge-ruby/rouge and https://github.com/highlightjs/highlight.js/ for the site (???) and https://microsoft.github.io/monaco-editor/monarch.html for for snippets and the Web IDE. Note that for Monarch, https://github.com/kdl-org/kdl-org.github.io already has monarch definitions.
• Forgejo, Gitea - https://github.com/alecthomas/chroma (I believe this is what they use, through goldmark and goldmark-highlighting)