feat: re-export transaction_builder

frontend/app_flowy/packages/flowy_editor/example/lib/plugin/image_node_widget.dart

@@ -1,4 +1,3 @@
-import 'package:flowy_editor/operation/transaction_builder.dart';
 import 'package:flowy_editor/flowy_editor.dart';
 import 'package:flutter/material.dart';
 

frontend/app_flowy/packages/flowy_editor/lib/flowy_editor.dart

@@ -6,5 +6,6 @@ export 'package:flowy_editor/document/path.dart';
 export 'package:flowy_editor/render/render_plugins.dart';
 export 'package:flowy_editor/render/node_widget_builder.dart';
 export 'package:flowy_editor/operation/transaction.dart';
+export 'package:flowy_editor/operation/transaction_builder.dart';
 export 'package:flowy_editor/operation/operation.dart';
 export 'package:flowy_editor/editor_state.dart';