Add Alejandra for Nix formatting

flake.nix

@@ -56,6 +56,8 @@
           devshells.default = {
             packages = with pkgs; [
               cowsay
+
+              alejandra
             ];
           };
 
@@ -84,9 +86,8 @@
 
           treefmt = {
             programs = {
+              alejandra.enable = true;
               mdformat.enable = true;
-              nixfmt.enable = true;
-              nixfmt.package = pkgs.nixfmt-rfc-style;
               prettier.enable = true;
               prettier.excludes = [ "*.md" ];
               stylish-haskell.enable = true;