Handle dhall-lsp-server crash

[PanAeon]

Server should be restarted on crash. Extra points if extension can remember number of times the server has crashed, stops restarting it after some threshold and displays a meaningful user message.

[EggBaconAndSpam]

This is already handled by VSCode's lsp implementation. This is implicitely mentioned in vscode-languageserver-node:

3.5.0 Server and Client

• allow the client to start the server in detached mode. If the server is running detached the client will not monitor the server process and kill it on shutdown.
• bug fixing.