Add set/get support for objects

[mrousavy]

Adds support for:

const user = {
  username: 'Marc',
  age: 20
}

MMKV.set(user, 'user')

and

const user = MMKV.get('user') // --> { username: 'Marc', age: 20 }

This is very WIP and I'm not sure if it even works since I think MMKV (protobuf) has to know the size of the object it should retrieve.

Closes #2

[mrousavy]

I think on android I have to store bytes manually 😧

This is how the flutter library does it: mmkv.dart#L300-L352 - note, they're also just storing bytes. I'll have to encode the jsi::Object into bytes first, then store it. Same for decoding. 🤔

[scarlac]

Dang ok. Sorry, I was hoping that the code was secretly ready on both platforms but figured there might be a good reason why it wasn't already integrated. Encoding the bytes essentially means reimplementing json.stringify again which is probably a bit much.

[kubanac95]

@mrousavy Not sure why people would not just JSON.strgify the object and store it like so. Is there any big performance improvements with the native approach?