Documentation + Nix fixes

[riz0id]

This PR contains several changes related to maintenance of Spectacle documentation, Haskell language server interoperability, and nix configuration changes. This PR builds off of the work in #52 which made nontrivial improvements to are documentation and Nix configuration.

• Nix changes:
  1. Adds a shell.nix for Haskell language server compatibility
  2. Factor out Nix overlays
• Haskell changes:
  1. Add show-extensions pragma to all Haskell modules
  2. Move internal Haskell modules to other-modules stanza
  3. Add haddock module descriptions to all Haskell modules
  4. Removed unused language pragmas
  5. Removed unused Haskell library modules and test modules
  6. Fix minor Haddock typos