Update to gitlab-ci 0.8.0 and horizon-spec 0.13.0
Summary
Updates horizon-gen-nix to use:
- gitlab-ci 0.8.0 with modular nix directory structure
- horizon-spec 0.13.0 (dhall spec 0.18.0)
- treefmt-nix instead of lint-utils
Changes:
- Created nix/systems.nix, nix/ci.nix, nix/packages.nix, nix/checks.nix, nix/impure-tests.nix, nix/treefmt.nix
- Added formatters: deadnix, statix, nixpkgs-fmt, cabal-fmt, dhall, hlint, stylish-haskell
- Added impure-tests as custom CI job
- Updated horizon-spec dependency to 0.13.0
- Updated test data to use dhall spec 0.18.0
- Removed .gitlab-ci.yml (now auto-generated by gitlab-ci 0.8.0)