feat: add rln-js and rln-identity examples

[weboko]

This PR introduces two examples:

• rln-js which showcases how chat can be done with RLN;
• rln-identity which shows how identity credentials can be managed;

rln-identity covers such things as:

• registering credentials;
• reading from keystore;
• importing / exporting;
• saving to localStorage;

rln-js covers:

• usage or credentials;
• sending / receiving messages with proofs;

Follow up PR will be to enable auto sharding and this is blocked for now by waku-org/js-rln#91