Create getPlatformElevation.windows.js (#212)

Fix "error: bundling failed: UnableToResolveError: Unable to resolve module `./getPlatformElevation` " to be compatible with react-native-windows

src/styles/getPlatformElevation.windows.js

+
+const getPlatformElevation = elevation => ({ elevation });
+
+export default getPlatformElevation;