Log file to record client errors

[akaroml]

Like language server logs, vscode-java can have a log file to record errors that happen at the client side.

Currently, users have to open the console to extract the logs as described in https://github.com/redhat-developer/vscode-java/wiki/Troubleshooting#get-the-vscode-java-logs

This can be improved by having a command "Java: Show language client log file" to make it easier and increase the possibility that users correctly provide the logs inside issues.

#782 can be improved by having log files.