Implemented rich inspect variables request

[JohanMabille]

@fcollonval the content of the debug request to send is:

{
    "type": "request", 
    "seq":  int, 
    "command": "richInspectVariables",
    "body": {
        "onBreakpoint", bool,
        "variableName": string,
        "variableReference", int
    }
}

The "onBreakpoint" field is temporary, I need to release xeus so I can get it out. For now, this only works when the debugger did not stop on a breakpoint. In this case, you can omit the variableReference field.

[fcollonval]

Thanks @JohanMabille
I'll update the JLab PR: jupyterlab/jupyterlab#10299

[JohanMabille]

@fcollonval you can now remove the onBreakpoint field in the request.