ReactDOM.useEvent: support custom types

[trueadm]

This PR adds support for custom user events to the useEvent system. I've also added tests demonstrating them working as expected. I also added an unnecessary phasedRegistrationNames == null check in accumulateTwoPhaseListeners that never occurs at runtime. This essentially allows this:

function Component() {
  const ref = useRef(null)
  const foo = ReactDOM.unstable_useEvent('foo')

  useEvent(() => {
    foo.setListener(ref.current, () => {
      console.log('Custom user events work!');
    });
  });

  return <span ref={ref}>;
}

const event = document.createEvent('Event');
event.initEvent('foo', true, true);
document. getElementsByName('span')[0].dispatchEvent(event);

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[threepointone]

approving in advance, with my usual nits

[threepointone]

I know this is old, but it's only obvious to me now that this isn't clear. What are the possible values of priority? Where can I learn more about it? etc. In a future PR this message should be made verbose, especially if we expect people to start using custom events.

[trueadm]

The possible values are 0, 1, 2 – for Discrete, user blocking and continious. I think we'll need to expose them from an enum on React maybe, but right now I'm unsure how that would work.