Add Plugins system to module

classes_extractor.info.yml

classes_extractor.services.yml

src/Annotation/ClassesExtractor.php

src/ClassesExtractorBase.php

src/ClassesExtractorManager.php

src/ClassesExtractorPluginInterface.php

src/ClassesExtractorPluginManager.php

src/Controller/ClassesExtractorController.php

src/Drush/ClassesExtractorCommands.php

src/Form/ClassesExtractorConfigForm.php

src/Plugin/ClassesExtractor/DsClassesExtractor.php

...ugin/ClassesExtractor/EditorFormatterClassesExtractor.php

...Plugin/ClassesExtractor/EntityDisplayClassesExtractor.php

src/Plugin/ClassesExtractor/SeedsLayoutClassesExtractor.php

src/Plugin/ClassesExtractor/SeedsPageClassesExtractor.php

src/Plugin/ClassesExtractor/ViewsClassesExtractor.php