chore: udpate textstyle_extensions version

frontend/app_flowy/pubspec.yaml

@@ -87,7 +87,7 @@ dependencies:
   charcode: ^1.3.1
   collection: ^1.16.0
   bloc: ^8.1.0
-  textstyle_extensions: ^1.1.0
+  textstyle_extensions: "2.0.0-nullsafety"
   shared_preferences: ^2.0.15
 
 dev_dependencies: