[hs] Set up new project with cabal/nix

hs2/.gitignore

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+/dist-newstyle

hs2/aoc-hs.cabal

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+cabal-version: 3.4
+-- Later versions are not supported by my current cabal2nix.
+
+name: aoc-hs
+version: 0.0.0.0
+build-type: Simple
+
+executable hs2
+    hs-source-dirs: app
+    main-is: Main.hs
+
+    build-depends:
+        base ^>=4.15.1.0
+
+    default-language: Haskell2010
+    ghc-options: -Wall

hs2/app/Main.hs

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+module Main where
+
+main :: IO ()
+main = putStrLn "Hello, Haskell!"

hs2/default.nix

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+{ pkgs ? import <nixpkgs> { } }:
+pkgs.haskellPackages.developPackage {
+  root = ./.;
+}