Consider adding a previous priority event property for priority change events

[shaseley]

This seems like it would be useful information, and isn't currently implemented. For example, something like:

let controller = new TaskController();  // user-visible priority
controller.signal.addEventListener('prioritychange', (e) => {
  console.log(e.previousPriority);  // user-visible
  console.log(e.currentTarget.priority) // background
});
controller.setPriority('background');