Support jupyterlab's debugger

[xpqz]

JupyterLab has great built-in support for interactive debugging, if the kernel supports it:

https://jupyterlab.readthedocs.io/en/stable/user/debugger.html

A kernel that wants to support this needs to implement jupyter's debugger protocol:

https://jupyter-client.readthedocs.io/en/latest/messaging.html#debug-request

..which is based on Microsoft's DAP: https://microsoft.github.io/debug-adapter-protocol/

This work (if completed) would potentially enable debugging also in VS Code, Emacs and vim, too (DAP-based).

Note that support for the DAP is proof-of-concepted here: https://github.com/tiamatica/vscode-apl-debug