Add ability to change json stringify / json parse methods for DataConnection

[afrokick]

Related to #582

By default, dataConnection uses JSON.parse and JSON.stringify methods when read/write data.

I added two fields to DataConnection:

DataConnection#stringify: (data: any) => string;

DataConnection#parse: (data: string) => any;

You can use, for example, https://github.com/WebReflection/flatted to support circular JSON parser.

import Peer from 'peerjs';
import { parse, stringify } from 'flatted/esm';

...

const dataConnection = peer.connect('123', { serialization: 'json' });

dataConnection.stringify = stringify;
dataConnection.parse = parse;

...

On another side:

import Peer from 'peerjs';
import { parse, stringify } from 'flatted/esm';

...

peer.on('connection', dataConnection => {
  dataConnection.stringify = stringify;
  dataConnection.parse = parse;
});

...

[kidandcat]

You should remove "json" from the methods name: stringify and parse, because with this change you can use any encode you want, not need to forcibly use JSON.