events: add once method to use promises with EventEmitter

This change adds a EventEmitter.once() method that wraps ee.once in a promise. I've been using this model for some months now, and it works extremely well for me.

const { once, EventEmitter } = require('events');
async function run() {
  const ee = new EventEmitter();
  process.nextTick(() => {
    ee.emit('myevent', 42);
  });
  const [value] = await once(ee, 'myevent');
  console.log(value);
  const err = new Error('kaboom');
  process.nextTick(() => {
    ee.emit('error', err);
  });
  try {
    await once(ee, 'myevent');
  } catch (err) {
    console.log('error happened', err);
  }
}
run();

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines