It's true that a software library would have to go under "other" / "everything else". (Whereas a Blender plugin in Python would go under Tools.) But you can link to a project's source code in the External Links tab, under Metadata. And there's nothing wrong with uploading a source code archive to Itch, either. Hope this helps!
Edit: come to think of it, Raylib is filed under Tools. And downloads can also consist of game demos, like in the case of Instead 3.