Expiration helpers

[peterszerzo]

Expose helpers to figure out the current expiration timestamp of a conversation, allowing the user to handle it however they wish. Alternative to #46.

const conversationHandler = nlxai.chatCore.create({ botUrl: "" });

conversationHandler.subscribe((responses) => {
  const timestamp = nlxai.chatCore.getCurrentExpirationTimestamp(responses);
  // Set timeouts, alerts etc.
});

[sam-trost]

looks like the right stuff to me.