configure nix fmt to use prettier

flake.nix

@@ -28,5 +28,42 @@
           };
         }
       );
+      formatter = forSystems (
+        system:
+        let
+          pkgs = pkgs' system;
+        in
+        pkgs.treefmt.withConfig {
+          runtimeInputs = with pkgs; [
+            nixfmt-rfc-style
+            nodePackages.prettier
+          ];
+          settings = {
+            tree-root-file = ".git/index";
+            formatter = {
+              nixfmt = {
+                command = "nixfmt";
+                includes = [ "*.nix" ];
+              };
+              prettier = {
+                command = "prettier";
+                options = [ "--write" ];
+                includes = [
+                  "*.css"
+                  "*.html"
+                  "*.js"
+                  "*.json"
+                  "*.jsx"
+                  "*.md"
+                  "*.mdx"
+                  "*.scss"
+                  "*.ts"
+                  "*.yaml"
+                ];
+              };
+            };
+          };
+        }
+      );
     };
 }