sourceReference for the source request

[jtpio]

At the moment we implement the source request from the Debug Adapter Protocol directly in xeus-python instead of forwarding the event to ptvsd, because it doesn't seem to be supported in interactive mode yet.

However the DAP requires sourceReference to be sent in the source request: https://microsoft.github.io/debug-adapter-protocol/specification#Requests_Source

So if we want to follow the DAP closely we should consider making the sourceReference field mandatory (even if it's not used).

[jtpio]

For reference the source request was added in https://github.com/QuantStack/xeus-python/pull/168.