Use Ninja by default on Windows

[friendlyanon]

CMake and Ninja come bundled since some version of VS2019 and has native support for CMake projects via "Open a local folder". I don't know the exact version and I'm too lazy to look up the Microsoft devblog that announced this.

MSBuild probably still has some niche usecases, but I'm not aware of any that would affect users of cmake-init.

Unless someone objects with reasons, I'll make Ninja Multi-Config the generator of choice for Windows soon-ish.