Experience Builder support

I think it is probably premature to add XB support in default content module, because the current XB data model is still unstable and under discussion. See 📌 Refactor the XB field type to be multi-valued, to de-jsonify the tree, and to reference the field_union type of the prop values Active and ✨ JSON-based data storage proposal for component-based page building Active but also other linked issues.

Core has a pluggable default content export system that will be released in 11.3.0. I already have an issue open for XB to integrate with that, so linking that here and closing this one as a duplicate. See https://www.drupal.org/node/3533854 → for more details about the core API.