Add checks in chatController before making API calls

[mliao95]

Issue #, if available:

Closes #129, #126

Description of changes:

This change adds safeguards to multiple ChatController methods to avoid unnecessary API calls.

• If the participant is disconnected and we attempt to call a method like sendMessage, the code will make an API call and get an error status.
  • To avoid this, we exit early if _participantDisconnected is true to avoid hitting the API call
• If the chat session has not yet connected, the code will display an ambiguous getConnectionToken is not a function error log
  • To avoid this, we check if connectionHelper is initialized before calling any methods that use getConnectionToken, and if it is not initialized, we log that connect() needs to be called before these APIs can work

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[mliao95]

LGTM, would this needed to be added to CHANGELOG for npm? Or we just copy/paste commit when publishing whenever the next publish happens

This change should ideally not impact the usage or functionality of chatjs, so copying and pasting the commit on next publish should be fine.