Add a pyp5js kernel backed by pyodide

[jtpio]

First steps towards #50 and #5.

TODO

• Load pyodide
• Handle basic python code (code evaluation, stdout / stderr)
• Add the pyp5js layer to setup the canvas
• Make kernel selection more generic (while keeping kernel specs hard-coded for now)
• Define a common interface for the two kernels. Most of the message handling can be shared, while keeping the "interpreter" specific to the kernel
• Layout a plan for a better integration with other custom kernels (for example as plugins)

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[jtpio]

Progress:

[jtpio]

Closing as this can be done by reusing the upstream Pyodide based kernel, as a follow-up to #72