Treat web workers correctly to use it as app or as library

[Raubzeug]

There are serious integration issues with the library's use of web workers: (1) CORS errors happen when worker scripts are not loaded from the same origin, (2) bundlers often require special handling for separate worker files, and (3) application developers are missing a way to configure the worker script path for their deployment setups.
Need to provide an explicit option to set the worker script URL at runtime, and add documentation explaining recommended bundling and deployment strategies for using workers with this library.

[Raubzeug]

After issue is done, should revert #3009

[artemmufazalov]

It should be done after #2236