addMainThread method
Adds the main thread (kind = main) of the text channel channelId, names it in the
channel's main_thread_id and returns its ID.
Implementation
String addMainThread(String channelId, {required String createdBy}) {
final id = addThread(channelId, 'main', createdBy: createdBy);
threads[id]!['kind'] = 'main';
channels[channelId]!.json['main_thread_id'] = id;
return id;
}