Add a pyp5js with pyodide kernel

[jtpio]

It would be nice to be able to switch between:

• a pure p5.js JavaScript kernel
• a Python kernel that exposes the p5 API, using pyp5js with pyodide, and everything still running in the browser

This would also be useful to prepare the ground for a tighter integration with Jupyter Widgets in a more traditional setup: jtpio/ipyp5#4