Describe the bug
If you have a private StatelessWidget that you'd like to turn into a StatefulWidget, when you using the VS Code helper to do so your new State class will be prefixed with an extra underscore:

For example, if your class name was _MyCustomWidget, when using Convert to StatefulWidget the new state generated will be__MyCustomWidgetState (notice the two underscores at the beginning)

To Reproduce

1. Create a private StatelessWidget
2. Click on Convert to StatefulWidget and see the new state being prefixed by 2 underscores.
   

Expected behavior
The new state generated only has one underscore.

Versions (please complete the following information):

• VS Code version: 1.56.2
• Dart extension version: 3.22.0
• Dart/Flutter SDK version: Flutter (Channel beta, 2.2.0-10.3.pre, on macOS 11.2.3 20D91 darwin-x64, locale en-US)