Fix worker bundle hoisting

[DeMoorJasper]

This is a branched of PR from #979

Main difference is that this PR adds options to the bundle that sets bundle.isolated, instead of looping through all dependencies every time you add a new one. (Should be slightly faster and more straightforward)
All credit goes to @dhcode for this, just wanted to bring some new life into this issue.

Closes #979

[owap]

Is this the magical line that will allow me to

import { debounce } from 'lodash';

...in my web workers? We're very keen on being able to bundle external assets in our webworker scripts

[DeMoorJasper]

This PR should fix it

[sebastiencs]

Thanks for your great work on parcel @DeMoorJasper

I have tested the branch fix/worker-bundle and I still get the error:
Uncaught Error: Cannot find module '99'
when I try to import a module in a worker:

import throttle from "lodash.throttle";

Am I missing something ?

[DeMoorJasper]

@sebastiencs that's strange could you create a minimal example repo to reproduce that?

This branch is far behind on the recent parcel version though, so it might actually have been fixed in the master if it's unrelated to this.

[sebastiencs]

@DeMoorJasper Turns out that if I change my worker filename from worker.tsx to worker.js it works, without changing anything else in the code. I will try to make a minimal repo to reproduce the error.