Add new command to clean the server workspace

[fbricon]

Instead of constantly pointing to the troubleshooting doc, we need a command that will:
1- display a warning (yada yada you sure ? y/n)
2- stop the Java server
3- delete the server workspace
4- reload the window.

1, 3, 4 are easy 2, not sure, hopefully not too hard either

[fbricon]

languageClient.stop(); should do the trick :-)
we may need to wait for the server to fully exit before deleting the workspace dir (looking at you file-locking Windows OS)