Add a tsserver commandline flag to enable logging

[mhegazy]

VSCode users need to collect server logs at times to help diagnose issues. The way this works today is by setting an environment variable which makes it hard to setup.

the proposal is to add --logFile and --logVerbosity to enable loggin on the server regardless of the env var; VSCode would start the server in this mode if it needs to collect logs, and it can control where the logs are stored as well.