refactor: move next to widgetbook

[YoussefRaafatNasry]

1. All widgetbook/next imports have been replaced by widgetbook/widgetbook.

   - import 'package:widgetbook/next.dart';
   + import 'package:widgetbook/widgetbook.dart';

2. Experimental badges have been removed.

3. Rename builders to story_builder and components_builder.

4. Builders are no longer opt-in, so the following code can be removed:

   - targets:
   -    $default:
   -       builders:
   -         widgetbook_generator:experimental_story_builder:
   -          enabled: true
   -        widgetbook_generator:experimental_components_builder:
   -           enabled: true

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