clear method
Implementation
void clear({String? channel}){
final targetChannel = channel ?? _GLOBAL_CHANNEL;
final observers = _observers[targetChannel] ?? [];
final observersInvoke = _observersInvoke[targetChannel] ?? Map();
observers.clear();
observersInvoke.clear();
_observers[targetChannel] = observers;
_observersInvoke[targetChannel] = observersInvoke;
}