Enhancement: API/unstable_dev should expose a fetch() function

[rozenmd]

From #1350 (comment):
It might be nice to hide the undici library, and also the underlying port, side of this and provide:

worker.fetch()

This method could take a path (or even a full URL if you wanted to simulate a custom route). This would also mitigate the case where port 8787 was not available and we ended up jumping on a different port, which I think we can do?