CancelRequest will be called on "$/cancelRequest".

Initialized is the callback that runs on the "initialized" method.

NewMethodHandler will instantiate a handler that contains callbacks to the server's methods.

NewSession instantiates a new session.

TextDocumentCompletion is the callback that runs on the "textDocument/completion" method.

MethodHandler is a struct that contains a logger, as well as methods to individual method handlers.

Service is a handler service for sessions.