Figure out a way to deliver messages to the client from the core layer

[Matsuuu]

There are situations in which we might want to inform the user (via a popup or other kind of a message), that there is something actionable to be done. Example of this would be that we notice that the user doesn't have a CEM set up for their project, and we might want to nudge them towards setting it up.

This could be done via some sort of a listener system where the server subscribes to some kind of a event from the core, and passes it onwards

Things to take into account:

• We might need custom handlers for all clients. So the client layer might need some extra code
• We don't want to create a circular dependency
• We want this to be low footprint, low maintenance and a surefire way to communicate things

[Matsuuu]

First part (core to server) done in https://github.com/Matsuuu/custom-elements-language-server/pull/26