Renaming some Files

[InsertCreativityHere]

This PR renames 3 of the files in the language server:

• slice_config.rs -> configuration.rs:
  This file holds two structs ServerConfig and SliceConfig. There's no reason for it to be named after only one of them...

• diagnostic_ext -> diagnostic_handler
  In rust, the ext suffix has a specific meaning: you're using the extension trait pattern to add functionality to a type.
  That isn't what this file does though, it just contains a handful of static helper functions.

• session -> server_state
  The current name gives the impression that this represents a single session between a client and server,
  but A) we don't even have 'session' semantics, and B) the actual fields it holds are:
  server_config and configuration_sets (which basically hold the data from a compiler run: Ast, diagnostics, etc).
  This sounds like server stuff more than session stuff. Also: we only construct one of these (ever), and it's constructed at server startup.