Issue #3498005 by fago: Render a tree of XB components into custom elements

.gitlab-ci.yml

README.md

composer.json

custom_elements.services.yml

...formatters/tests/src/Kernel/TableFieldCeFormatterTest.php

src/CustomElement.php

src/CustomElementsBlockRenderHelperTrait.php

src/RenderConverter/ExperienceBuilderRenderConverter.php

tests/src/Functional/CustomElementsRenderMarkupTest.php

tests/src/Kernel/EntityReferenceCeFieldFormatterTest.php

tests/src/Kernel/ExperienceBuilderIntegrationTest.php

tests/src/Kernel/Formatter/ImageCeFieldFormatterTest.php

tests/src/Kernel/Formatter/PathCeFieldFormatterTest.php

tests/src/Kernel/ThunderParagraphRenderMarkupTest.php

tests/src/Traits/TestDebugHelperTrait.php

tests/src/Traits/TestHelperTrait.php