Syntax highlighting for IDL

[Falco]

Discourse leverages Highlight.js for this work, which supports 191 languages, but I can’t find IDL on the list.

May be helpful to check the library and how you can add support for IDL on it:

We're not a CDN - highlight.js

There are some issues mentioning IDL:

https://github.com/highlightjs/highlight.js/pull/1315