Added AppVeyor support

This adds another continuous integration service: https://ci.appveyor.com/project/OpenRA/openra/ that is free for Open Source projects. It runs the build on Windows machines using .NET to test our IDE/msbuild project files and the PowerShell scripts. It also runs the NUnit tests from #6797 and stores the results. Yes, some of them are still failing. This may urge us to finally fix the MiniYAML merge code. Also some people had trouble running them https://github.com/OpenRA/OpenRA/pull/6984 so this should simplify it.
Super-nit: it's spelled 'continuous'