Display custom language source folder similar to java source folder

Hello!

I'm trying to display my custom language's directory structure similar to the way java src files are shown, but I'm having trouble finding an API to do this. If needed I could just add my source folder as a java source folder from the accompanying gradle plugin, but if possible I'd like it to be part of the IDE plugin.

Right now my project looks like
src
--main
----java (in blue)
------com.sample
--------package1
----------Example.java
--------package2
----------Example2.java
----custom_lang (not blue)
------com
--------sample
----------package1
------------Example.sample
----------package2
------------Example2.sample

I'd like to have the custom_lang folder behave like java does (display in blue, collapse packages)

Thank you!