♻️⚡️ move core to a SharedWorker

[jxn-30]

SharedWorkers are a fancy concept to allow multiple browser windows/tabs (and iframes) of the same origin to use a script that lives on another Thread.

I think, we could improve V4 Performance a lot if core lives in a SharedWorker managing all the shared data and iframes and other window-instances can use this core rather than managing this data themselfes. This would also make the use of broadcasting for data sharing obsolete.