Add support for WebWorker with worker-plugin

[Krivega]

Closes#3660

I used to test a simple example of the testplugin

// index.js
const worker = new Worker('./worker', { type: 'module' });
worker.onmessage = ({ data }) => {
console.log('page got data: ', data);
};
worker.postMessage('hello');

// worker.js
import { foo } from './dep';
console.log('hello from worker');
addEventListener('message', ({ data }) => {
console.log('worker got message', data);
if (data === 'hello') {
postMessage(foo);
}
});

// deep.js
export const foo = 'bar';

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[iansu]

There is an existing PR that's very similar here: #3934

In the thread for #3660 there were some issued raised about browser compatibility with worker-plugin which I think need to be investigated.

[Krivega]

@iasu right, I'll try to fix it in the plugin and then try again