LibWeb: Implement `WorkerGlobalScope.importScripts()`

[tcl3]

This method allows workers to synchronously import one or more scripts. It is used by all *.worker.html WPT tests. Before this change all of these tests would time out, so this should yield a significant reduction in test timeouts.

Currently set to draft, as my implementation currently spins forever (most of the time) while waiting for a HTTP fetch request to complete.