feat: add builder addon

[yousinix]

The UseCaseBuilderAddon was added in #880. It's now replaced with a simpler alternative called BuilderAddon that shows no fields on the side panel, compared to a boolean field for the UseCaseBuilderAddon.

The new addon can be used as follows:

BuilderAddon(
    name: 'Red',
    builder: (context, child) => ColoredBox(
        color: Colors.red,
        child: child,
    ),
)