Make it run in github codespaces

[Simon-Laux]

gh codespaces converts ports to urls.

example:

https://laughing-space-happiness-g4vgrp6xw5pf9r4g-7001.app.github.dev/

I already did it for stackblitz in #58, so should be doable.

[Simon-Laux]

I started an attempt in #80, but I got stuck. Some time passed, maybe it works better meanwhile, or we could explore different approaches to webxdc instance isolation and communication like what https://gitlab.com/soapbox-pub/webxdc.app is doing with subdomains and service workers.