Add support for WebWorker with worker-loader (#3660)

Created by: iansu

Use worker-loader to turn any file that ends with .worker.js into a WebWorker.

Closes #3660

Here is the sample WebWorker code I used to test this:

// hello.worker.js

let helloInterval;

const sayHello = () => {
  self.postMessage({ message: 'Hello' });
};

self.addEventListener('message', event => {
  if (event.data.run === true) {
    self.postMessage({ status: 'Worker started' });
    helloInterval = setInterval(sayHello, 1000);
  }

  if (event.data.run === false) {
    self.postMessage({ status: 'Worker stopped' });
    clearInterval(helloInterval);
  }
});

// index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';
import HelloWorker from './hello.worker.js';

ReactDOM.render(<App />, document.getElementById('root'));

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: http://bit.ly/CRA-PWA
serviceWorker.unregister();

const helloWorker = new HelloWorker();
let messageCount = 0;

helloWorker.postMessage({ run: true });

helloWorker.onmessage = event => {
  if (event.data.status) {
    console.log('STATUS', event.data.status);
  }

  if (event.data.message) {
    messageCount += 1;
    console.log('MESSAGE', event.data.message);

    if (messageCount >= 5) {
      helloWorker.postMessage({ run: false });
    }
  }

}