connection.sendNotification for init/exit events

First of all, thanks for all the awesome work in this repository.

In `graphql-language-service-server` we can send `init`/`exit` and other notification events from server to client to easily build features like status bar in an editor like VSCode.

Similarly to [`graphql-for-vscode`](https://github.com/kumarharsh/graphql-for-vscode/blob/master/src/client/extension.ts) where they have created a status bar as they have built a custom LSP server on top of [gql service](https://github.com/Mayank1791989/gql).

Opinion on this or suggested alternatives?

If we can lock on an API, I can take up the implementation part.

Thanks!