Add `dhall format`

[srid]

Resolves #49

Relies on https://github.com/dhall-lang/dhall-haskell/pull/2169

[bors[bot]]

Build succeeded:

• check alejandra [x86_64-linux]
• check black [x86_64-linux]
• check buildifier [x86_64-linux]
• check cabal-fmt [x86_64-linux]
• check dhall [x86_64-linux]
• check elm-format [x86_64-linux]
• check gofmt [x86_64-linux]
• check gofumpt [x86_64-linux]
• check hclfmt [x86_64-linux]
• check hlint [x86_64-linux]
• check mdsh [x86_64-linux]
• check nixfmt [x86_64-linux]
• check nixpkgs-fmt [x86_64-linux]
• check ocamlformat [x86_64-linux]
• check ormolu [x86_64-linux]
• check prettier [x86_64-linux]
• check purs-tidy [x86_64-linux]
• check rufo [x86_64-linux]
• check rustfmt [x86_64-linux]
• check scalafmt [x86_64-linux]
• check shellcheck [x86_64-linux]
• check shfmt [x86_64-linux]
• check stylish-haskell [x86_64-linux]
• check stylua [x86_64-linux]
• check terraform [x86_64-linux]
• check testBorsToml [x86_64-linux]
• check testEmptyConfig [x86_64-linux]
• check testExamples [x86_64-linux]
• check testWrapper [x86_64-linux]
• Evaluate flake.nix