Add formatter

formatter.nix

@@ -0,0 +1,27 @@
+{
+  self,
+  nixpkgs,
+  treefmt-nix,
+  flake-utils,
+  pre-commit-hooks,
+  ...
+}:
+flake-utils.lib.eachDefaultSystem (
+  system: let
+    pkgs = nixpkgs.legacyPackages.${system};
+    treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
+    treefmtWrapper = treefmtEval.config.build.wrapper;
+  in {
+    packages.formatter = treefmtWrapper;
+    formatter = self.packages.${system}.formatter;
+    checks.pre-commit-check = pre-commit-hooks.lib.${system}.run {
+      src = ./.;
+      hooks = {
+        treefmt = {
+          enable = true;
+          package = treefmtWrapper;
+        };
+      };
+    };
+  }
+)