Support for Shared Web Workers

[Shared Web Workers](https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker) are a relatively new feature that allow multiple scripts communicate with a single worker. A good use case would be for example sharing computation between multiple browser tabs.

When it comes to [browser support](https://caniuse.com/sharedworkers), the feature is well supported apart from Safari. It used to have the feature for a while but then it was dropped. The background for the decision is explained at https://bugs.webkit.org/show_bug.cgi?id=149850 .

As we would have a direct use case for the feature in our current project, would it be possible for you to consider inclusion of the feature to threads.js? I am happy to contribute the code if we can agree on the steps. I imagine we would have to expose `SharedWorker` from **threads.js** itself and then also make sure the addition works with the webpack plugin. Safari might require some kind of a strategy as well.

Reference: https://github.com/GoogleChromeLabs/worker-plugin/pull/42 (shared worker support in Google's worker-plugin)