The generated widget_preview_scaffold project needs to explicitly
reference the assets from the parent project's pubspec.yaml. This change
updates flutter widget-preview start to read the parent project's
pubspec.yaml and add references to the assets listed to the
widget_preview_scaffold's pubspec.yaml. If generate: true is set in the
parent project, a reference to the autogenerated flutter_gen package is
manually added to the widget_preview_scaffold's package_config.json.
