Issue #3492349 by berdir: Support attributes for plugin discovery

.gitlab-ci.yml

...c/Plugin/paragraphs/Conversion/ParagraphsDemoUserText.php

...in/paragraphs/Conversion/ParagraphsDemoUserToTextUser.php

...ugin/Field/FieldFormatter/LibraryItemSummaryFormatter.php

...ragraphsLibraryItemHasAllowedParagraphsTypeConstraint.php

src/Attribute/ParagraphsBehavior.php

src/Attribute/ParagraphsConversion.php

src/Entity/Paragraph.php

src/Entity/ParagraphsType.php

src/ParagraphsBehaviorManager.php

src/ParagraphsConversionManager.php

src/Plugin/EntityReferenceSelection/ParagraphSelection.php

...lugin/Field/FieldFormatter/ParagraphsSummaryFormatter.php

src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php

src/Plugin/Field/FieldWidget/ParagraphsWidget.php

...t/src/Plugin/paragraphs/Behavior/TestBoldTextBehavior.php

...test/src/Plugin/paragraphs/Behavior/TestDummyBehavior.php

...lugin/paragraphs/Behavior/TestFieldsSelectionBehavior.php

.../src/Plugin/paragraphs/Behavior/TestTextColorBehavior.php

...n/paragraphs/Conversion/ParagraphsTestTextToTextImage.php