Set up Windows CI, with artifacts

This is the most important task now so that Windows players can get updates whenever bugs are fixed, etc.

Unfortunately, haskell-ci still doesn't do Windows and, anyway, we also need publishing artifacts so that players can get them. But there surely are examples of such CI Haskell setups on github. An old but working one is https://github.com/haskell/cabal/blob/master/.github/workflows/validate.yml

Our old appveyor setup, with commands to create installer files, etc., is at https://github.com/LambdaHack/LambdaHack/blob/master/appveyor.yml. It failed, in part, because I needed old versions of SDL, from before it got broken, and Windows package management (packman was it?) doesn't keep old versions. I hope current SDL, as available on Windows, has the font and input loop errors fixed and the game runs. If not, I will step in and try to work around the bugs (again).

Start by comparing the old appveyor.yml installer commands with the Haskell CI example at .github/workflows/validate.yml. Check how current SDL is obtained on Windows and how the game is built, then set up Windows CI that publishes installer artifacts players can download and verify.