Basic GH CI for Windows

[cmb69]

While there is already an AppVeyor CI, the problem with that is that usually only one job runs in parallel, but we want to be able to run the builds for multiple PHP versions.

For now, we just run for PHP 7.4, though, since some stuff needs to be fixed, e.g. #13, and the failing tests[1].

[1] https://github.com/cmb69/php-rar/runs/4370012547