Bring across VIRTUAL_PATH support from the jwilder proxy

[far-blue]

The jwilder proxy has recently added support for VIRTUAL_PATH to allow sub-path routing. It would be hugely useful for my team if that support was added to the codekitchen proxy.

[codekitchen]

👍 more generally, I wish dinghy-http-proxy was just using the jwilder proxy directly rather than forking it. I haven't looked at the jwilder proxy in a few years, maybe it'd be possible now to switch to it.