Use jupyterlite-xeus

[jtpio]

Fixes https://github.com/jupyterlite/repo2jupyterlite/issues/17

Using https://github.com/yuvipanda/environment.yml/pull/1 for testing.

• [x] Switch to jupyterlite-xeus
• [x] Document how to install create the environment.yml (which can be a note in the README for now)

[jtpio]

Looks like this is working fine with https://github.com/yuvipanda/environment.yml/pull/1:

binderlite-jupyterlite-xeus.webm