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;
+}

flake.lock

@@ -76,7 +76,28 @@
       "inputs": {
         "flake-parts": "flake-parts",
         "nixos-generators": "nixos-generators",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1689070229,
+        "narHash": "sha256-99VU2FTkEdO3/1Qr78fHWWlN5GaOGLaXDi26PNiUf+I=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "3c54278bf7b8642eba174a22ca02d5552c21dc0b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
       }
     }
   },

flake.nix

@@ -7,6 +7,8 @@
     nixos-generators.inputs.nixpkgs.follows = "nixpkgs";
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
   };
 
   outputs = inputs@{ flake-parts, ... }:
@@ -14,6 +16,7 @@
       systems = lib.systems.flakeExposed;
       imports = [
         ./flake-parts/packages.nix
+        ./flake-parts/formatting.nix
       ];
       flake = {
         nixosConfigurations.installer = lib.nixosSystem {