There is an official VSCode extension for IDL! NV5 Geospatial mentions it in a blog post:
TLDR: We have an official extension for Visual Studio Code called IDL for VSCode! It is open source on GitHub, you can see some of the features we have planned, and there’s a place to ask questions and provide feedback. See below for a story about our journey to get here, our inspiration for the user experience, and our goals for community engagement.
The extension seems fairly useful. It’s more than just a syntax highlighter. It can give linting-type information: finding syntax errors, give recommendations (for example, you are missing a
compile_opt statement), warnings like unused variables, etc. And, of course, everyone’s favorite editor feature — auto-completion.
There is a, so far, quiet, GitHub Discussion for discussing the extension.