support react-sketchapp

[karlsander]

This is not a polished PR intended to be merged but just the quick fork I made for myself. I created the PR to ask how interested you are in adding this. I would be willing to work on a more serious solution too. For now I:

• changed react-native to react-primitives so StyleSheet.create and Platform can work: seems like a decent solution but adding a dependency to this otherwise dependency free project is not so great. Maybe people usually cover this with webpack aliasing in their react-sketchapp setups anyway? How is it handled for react-native-web?
• copied all the .ios.js files to .sketch.js files: likely not exactly the right solution but close enough for me