Add `SharedWorker` to the library

[mhegazy]

Details in https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker

This should work

var myWorker = new SharedWorker('worker.js');

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[shamhub]

@mhegazy what about service worker?

[mhegazy]

Can you file a new one for these? we need a new lib.serviceworker.d.ts as well.

[Codetoil]

For some reason, SharedWorker exists in the DOM lib, but not in WebWorker lib.