Building releases as standalone files

OK, so finally got to make single-file releases of our game. No more dealing with whole repo content, creating virtual environment and all that stuff.

The game is written in Python and PyGame, so we are now using PyInstaller. Using GitHub actions job with a matrix strategy, we are now able to automatically build releases for 3 major operating systems. It feels very clean.

I need to work on adding ARM build for MacOS, adding version (git tag) to artefact names and creating automated GitHub releases, but that is a future.

Please let us know if Windows and MacOS builds work, we only tested Linux build :-)