JupyterLab debugger extension

[afshin]

Background

We have had a number of conversations about adding variable inspection and code debugging tools to JupyterLab since the earliest stages of JupyterLab's development. This is in line with creating an opt-in set of tools to provide more IDE-like environment for users who want to dive into the code.

Current status

The emerging standard in this space has been the Debug Adapter Protocol, which we have been adopting in Jupyter projects, cf. @JohanMabille's work on jupyter/jupyter_client#446 and the pull requests linked in that issue.

Next steps

I propose we create a jupyterlab/debugger repository for a JupyterLab extension that provides a user interface for debugging tools within JupyterLab.

Full disclosure - I work at Two Sigma and we are interested in this space and working on this functionality in the coming months.

[SylvainCorlay]

+1 on this.

[JohanMabille]

I have added back the debug messages in jupyterlab/jupyterlab#6704. Even if this PR should not been merged before the protocol is updated, the branch can be used to test the JupyterLab extension providing a user interface for debugging.