Add claim option to register()

Currently if a first page load wants to register a service worker and immediately be controlled by it it has to do quite a bit of boilerplate:

// serviceworker.js
self.addEventListener('activate', (event) => {
  event.waitUntil(clients.claim());
});

// main.js
async function registerServiceWorker() {
  await navigator.serviceWorker.register('serviceworker.js');
  const registration = await navigator.serviceWorker.ready;
  const worker = registration.active;
  if (worker.state === 'activated') {
    return;
  }
  await new Promise(resolve => {
    worker.addEventListener('statechange', () => {
      if (worker.state = 'activated') {
        resolve();
      }
    });
  });
}

We could simplify this by adding a new option to register that does the same thing (including the SW side) and reduce all this down to:

// main.js
await navigator.serviceWorker.register('serviceworker.js', { claim: true });

Start with the navigator.serviceWorker.register() entry point and compare the serviceworker.js and main.js examples in the issue. Determine the API behavior required for { claim: true }, including the service-worker-side activation and claiming flow. Done means the option provides the described immediate-control behavior without the shown boilerplate.