Support window/logMessage

[krassowski]

What are you trying to do?

Some servers provide useful messages to the client. We should show these in the built-in JupyterLab console or in the browser console at the very minimum.

{"jsonrpc":"2.0","method":"window/logMessage","params":{"type":3,"message":"No pyproject.toml file found."}
{"jsonrpc":"2.0","method":"window/logMessage","params":{"type":2,"message":"stubPath typings is not a valid directory."}}
{"jsonrpc":"2.0","method":"window/logMessage","params":{"type":3,"message":"Assuming Python platform Windows"}}
{"jsonrpc":"2.0","method":"window/logMessage","params":{"type":3,"message":"Searching for source files"}}
{"jsonrpc":"2.0","method":"window/logMessage","params":{"type":3,"message":"No source files found."}}

https://microsoft.github.io/language-server-protocol/specification#window_logMessage

The idea would be to use ILogger from https://github.com/jupyterlab/jupyterlab/tree/master/packages/logconsole