Add JupyterLab debugger support

[lassoan]

xeus-python kernel supports debugging, but it it currently not supported in the Slicer kernel.

The issue is that the application hangs when the debugger is attached, because a message that comes in the control channel would need to be processed in the main thread, but Slicer processes all messages (including the ones coming from the control channel) on the main thread.

See more information here: jupyter-xeus/xeus-python#275