Set up post-commit automatic builds via Github Actions

[slime73]

It'd be cool if we had a Github Actions script to make builds on every supported platform after each commit, to help check commits (and pull requests) for compile issues. Auto-uploading the builds as part of that script would be handy too.

Not every platform would need to be added at once.

Here's an example from one of my other projects - it uses different build tools but the idea is the same.

[uberlinuxguy]

Platforms:

• Linux
• Windows
• Mac

[uberlinuxguy]

Windows builds happen now. An artifact is uploaded too, with an installer if anyone wants to test the main branch builds.