gc: add finalization for URL

[guybedford]

This is a first attempt to fix https://github.com/bytecodealliance/StarlingMonkey/issues/101 for further feedback.

In testing, I'm still not able to get the finalizer to run, as I believe we disable the GC during runtime still.

That said, supporting the GC running for long-running executions, and coding to avoid memory leaks still seem like important properties to retain for the runtime in general, even if not used.

@tschneidereit would very much value your feedback further here.