feat(Page): introduce workers (#2560)

This adds `page.workers()`, and two events `workercreated` and `workerdestroyed`. It also forwards logs from a worker into the page `console` event.

Only dedicated workers are supported for now, ServiceWorkers will probably work differently because they aren't necessarily associated with a single page.

Fixes #2350.

test/assets/worker/worker.js

@@ -0,0 +1,16 @@
+console.log('hello from the worker');
+
+function workerFunction() {
+  return 'worker function result';
+}
+
+self.addEventListener('message', event => {
+  console.log('got this data: ' + event.data);
+});
+
+(async function() {
+  while (true) {
+    self.postMessage(workerFunction.toString());
+    await new Promise(x => setTimeout(x, 100));
+  }
+})();