flake-module: enable treefmt as default formatter for the flake

Author: Mic92

flake-module.nix

@@ -27,11 +27,20 @@ in
             ```
           '';
           type = types.submoduleWith {
-            modules = (import ./.).all-modules pkgs;
+            modules = (import ./.).all-modules pkgs ++ [{
+              options.flakeFormatter = lib.mkOption {
+                type = types.bool;
+                default = true;
+                description = lib.mdDoc ''
+                  Enables `treefmt` the default formatter used by the `nix fmt` command
+                '';
+              };
+            }];
           };
         };
         config = {
           checks.treefmt = config.treefmt.build.check self;
+          formatter = lib.mkIf config.treefmt.flakeFormatter (lib.mkDefault config.treefmt.build.wrapper);
         };
       });
   };