refactor(flake): wrap xonsh as a package

shells/default.nix

@@ -0,0 +1,49 @@
+{inputs, ...}: {
+  imports = [
+    inputs.git-hooks-nix.flakeModule
+  ];
+
+  perSystem = {
+    config,
+    pkgs,
+    ...
+  }: {
+    pre-commit = {
+      check.enable = true;
+      settings.hooks = {
+        alejandra.enable = true;
+        nil.enable = true;
+        shellcheck.enable = true;
+        prettier.enable = true;
+        ruff.enable = true;
+        ruff-format.enable = true;
+        shfmt.enable = true;
+        deadnix = {
+          enable = true;
+          args = ["-e"];
+        };
+        statix = {
+          enable = true;
+          args = ["fix"];
+        };
+      };
+    };
+
+    devShells = {
+      default = config.pre-commit.devShell;
+      haskell = pkgs.mkShell {
+        name = "haskell";
+        packages = builtins.attrValues {
+          inherit (pkgs) ghc;
+          inherit
+            (pkgs.haskellPackages)
+            stack
+            haskell-language-server
+            ormolu
+            ;
+        };
+      };
+    };
+    formatter = pkgs.alejandra;
+  };
+}