elm: add elm related binary

Author: smoothprogrammer

template/elm/flake.nix

@@ -25,6 +25,9 @@
             buildInputs = with pkgs; [
               editorconfig-checker
               elmPackages.elm
+              elmPackages.elm-language-server
+              elmPackages.elm-test
+              elmPackages.elm-format
             ];
           };
         }