Support multiple MMKV instances

Example:

const MMKV1 = new MMKVLoader()
  .withInstanceID('test1')
  .withEncryption()
  .initialize();

const MMKV2 = new MMKVLoader()
  .withInstanceID('test2')
  .withEncryption()
  .initialize();

if (__DEV__) {
  mmkvFlipper([MMKV1, MMKV2]);
}