support registering hook in worker threads

[kherock]

This provides a better implementation for #285 by setting and reading the main script from process.env. This allows modules required from threads to trigger reloads.

I considered some alternative solutions where we monkeypatch into the Worker constructor, but ultimately I think an env variable solves this well

[kherock]

Hi @bjornstar! Could this get a review please?

[bjornstar]

I'd like to see a test so we can validate it's working.

[kherock]

After adding a test case, I realized that it wasn't actually working as I expected - worker threads don't seem to inherit IPC handles, so I had to adapt the ipc.js helper to set up a new MessagePort as well.