Github Actions file

[starseeker]

I'm not quite sure how this would work as a pull request, or whether you might need to commit it directly yourself to have it set up properly, but I put together a basic Github Actions file for detria that will compile and run the tests on Windows, Linux, and OSX automatically for each push:

https://github.com/starseeker/detria/blob/master/.github/workflows/check.yml

If that's of interest (might help to catch regressions more quickly) please feel free to use it.