Windows support

Would be nice to have support for react-native-windows.

I've already started working on this at the support-windows branch, but hit a roadblock because I couldn't add the MMKV/Core/core.vcxproj to my solution since that's VS 2015 and apparently my VS 2019 can't read that:

No idea how to add that without touching the submodule, but if anyone has some ideas let me know!