Return a promise in "once" if no listener provided

[mourner]

Closes #10192. Now evented.once(eventName) now returns a promise if no listener function is provided (thanks @stevage for the suggestion!), allowing for ergonomic usage of async/await with GL JS while not breaking backwards compatibility:

(async () => {
  const map = new mapboxgl.Map(...);
  await map.once('load');
  map.addLayer(...);
  await map.once('idle');
})();

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog></changelog>

[ryanhamley]

This works well and is a nice incremental improvement. I just have a nit with the doc string.