feat(fmt): Add formatter using alejandra and format flake

flake.nix

@@ -38,6 +38,10 @@
 
     # This is where the Neovim derivation is built.
     neovim-overlay = import ./nix/neovim-overlay.nix {inherit inputs;};
+    inherit (self) outputs;
+    # This is a function that generates an attribute by calling a function you
+    # pass to it, with each system as an argument
+    forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
   in
     flake-utils.lib.eachSystem supportedSystems (system: let
       pkgs = import nixpkgs {
@@ -66,6 +70,7 @@
         '';
       };
     in {
+      formatter = nixpkgs.legacyPackages.${system}.alejandra;
       packages = rec {
         default = nvim;
         nvim = pkgs.nvim-pkg;