feat: Add `MMKV.config` getter

[mrousavy]

Adds MMKV.config to get the original config used to create this MMKV instance.

const storage = createMMKV({ id: 'custom' })
const id = storage.config.id // <-- custom

• Resolves https://github.com/mrousavy/react-native-mmkv/issues/941