Separating signal from Observer

[petamoriken]

suggestion:

dictionary Observer {
  ObserverCallback next;
  VoidFunction complete;
  ObserverCallback error;
};

dictionary ObserverOptions {
  AbortSignal signal;
};

[Exposed=*]
partial interface Observable {
  undefined subscribe(optional Observer observer = {}, optional ObserverOptions options = {});
};

This issue was split from #71