I didn’t know that Moodle was offering that kind of plugin – we also set up our own grading system from the ground up, using a dedicated submission web interface, a dedicated maven script to compile student projects (exported from ScalaIDE), and a home-made script to extract results of unit tests and scalastyle messages.
I looked up the plugin (I think) you mention, and it seems to be documented here https://coderunner.org.nz/
I have no idea of how it works. Coderunner extension seems to be documented here : https://github.com/trampgeek/moodle-qtype_coderunner/blob/master/Readme.md#supporting-or-implementing-new-languages