Re-organize project structure

[eiriktsarpalis]

Originally posted by @twenzel in PederHP/mcpdotnet#52.

As started with a discussion in PederHP/mcpdotnet#30 (comment) we should re-think about how the project is structured.

Should there be a core library, a client library, and a server library?
Should the Server library depend on Microsoft.Extensions.Hosting? Or might there be an extended Server library?