added formatter

flake.nix

@@ -5,12 +5,16 @@
     nixpkgs.url = github:nixos/nixpkgs/nixos-23.05;
   };
 
-  outputs = { self, nixpkgs }:
-    let
+  outputs = {
+    self,
+    nixpkgs,
+  }: let
     supportedSystems = ["x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin"];
     forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
     pkgs = forAllSystems (system: nixpkgs.legacyPackages.${system});
   in {
+    # Formatter
+    formatter = forAllSystems (system: nixpkgs.legacyPackages.${system}.alejandra);
 
     # Packages
     packages = forAllSystems (system: {
@@ -26,7 +30,6 @@
           (poetry2nix.mkPoetryEnv {projectDir = self;})
           poetry
         ];
-
       };
     });