Add auto-build via Github actions

[jasonswearingen]

It would be nice to have an automated build process like what raylib-cs has. This would automatically build the windows,linux,osx binaries when a code change is commited.

Currently building for Windows and Linux can be done manually, which is good enough for me in the medium term, which is why I can't prioritize working on it myself for now.

Update: it seems raylib-cs doesn't actually build, it just coppies the native raylib binaries. So an actual github action to build+bundle extras seems like it would be a lot more work!