chore: add module for formatting

This will export a check for formatting and makes `nix fmt` use treefmt

flake-parts/formatting.nix

@@ -0,0 +1,15 @@
+{self, inputs, ...}: {
+  imports = [
+    inputs.treefmt-nix.flakeModule
+  ];
+  perSystem = {
+    pkgs,
+    ...
+  }: {
+    treefmt.projectRootFile = "flake.nix";
+    treefmt.flakeCheck = true;
+    treefmt.flakeFormatter = true;
+    treefmt.programs.alejandra.enable = true;
+    treefmt.programs.shellcheck.enable = true;
+  };
+}

flake-parts/pre-commit.nix

@@ -0,0 +1,4 @@
+{ self, ... }: {
+  pre-commit.settings.hooks.alejandra.enable = true;
+  pre-commit.settings.hooks.shellcheck.enable = true;
+}