Break apart lib.rs

[Marwes]

lib.rs is about to be over 3k lines with #79 so we should really break it up in a few submodules. Might make sense to break it up roughly in the sections in https://microsoft.github.io/language-server-protocol/specification maybe?

[Marwes]

Also, do we want re-export all the types currently in lib.rs after they are moved?

[matklad]

FWIW, as a user of langaugeserver_types, I slightly prefer everything to be in one single file: it's easier to search for things when browsing source.

[kjeremy]

I think this is worth revisiting. I've struggled to catch everything when adding (and specifically modifying) types for new protocol versions.