V2: Include PDF.js web-worker in a way that works with bundling

[jzillmann]

Right now following pieces are in place:

• In snowpack.config.js

  mount: {
        ...      
        'node_modules/pdfjs-dist/es5/build/': { url: '/worker', static: true },
    },

• And then in ui/src/store.js

pdfjs.GlobalWorkerOptions.workerSrc = `worker/pdf.worker.min.js`;

This works in dev mode, but for a production deployment (npm run build) one has to copy node_modules/pdfjs-dist/build/pdf.worker.min.js to worker/pdf.worker.min.js.

There has to be a better way!

There is related documentation and conversation:

• https://www.snowpack.dev/guides/web-worker/#nav-primary
• documentation: Web Worker Support FredKSchott/snowpack#1280
• Pain points while migrating over a large project FredKSchott/snowpack#1387

But i failed so far with any approach...

[darkcheftar]

I guess current setup fixes #28