chore: add module for formatting

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

flake-parts/pre-commit.nix

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