feat: support customizing the mobile magnifiter (#625)

* feat: support customizing the mobile magnifiter

* chore: remove unused test

* feat: expaned the padding between cursor and toolbar

* feat: support long pressing to update selection

* chore: revert code sign

example/lib/pages/mobile_editor.dart

@@ -123,6 +123,8 @@ class _MobileEditorState extends State<MobileEditor> {
         ),
       ),
       padding: const EdgeInsets.symmetric(horizontal: 24.0),
+      magnifierSize: const Size(144, 96),
+      mobileDragHandleBallSize: const Size(12, 12),
     );
   }