Configure Nix shell dev environment

It should be possible to configure a Dev env shell with one command (I'm thinking about: https://devenv.sh/) that brings into the scope:

- Entire required OCaml toolchain
- All dependencies
- All dev tools
  - `ocamlformat`
  - `ocaml-lsp`
  - `utop`
- Required runtime dependencies
  - `bat`
  - (Optional) Fonts