Syntax highlighting support for custom languages

[glaebhoerl]

General feature request: The ability to specify syntax highlighting in code blocks for a given language that isn't already supported out of the box.

Our own particular use case: We'd like to use MyST to generate the user-facing documentation for our own custom domain-specific language. We don't have any special constraints or requirements: even just basic lexical highlighting of e.g. keywords and literals would be adequate and much appreciated. Just so that it's nicer to look at and easier to scan than plain monochromatic text. At the technical level, likewise, anything at all would be fine as long as it doesn't require forking upstream projects: whether it's as a MyST-level feature, plugin support, or some low-level "escape hatch" that enables directly interfacing with the underlying syntax highlighting library actually being used. (Which is currently highlight.js, IIUC; and which supports a .registerLanguage() API... if only I had some way to invoke it!)

[agoose77]

Thanks for opening this issue!

Related topics to keep an eye on for people tackling this: #1715 #1683