Structured Block Kit fields: portable_text and block_list, plus Duplicate on plugin block cards

[simone-coelho]

Plugin blocks that carry rich text or nested content currently have no faithful Block Kit field for it. A rich-text value stored as Portable Text can only surface through a multiline text_input, which shows an empty box over the stored array and replaces it with a plain string on save — silent data loss for any content imported or authored as Portable Text. Similarly, a block whose data contains an ordered list of other registered blocks (columns, composite sections) has no field kind the dialog can edit, and plugin block cards offer Edit/Delete but no Duplicate.

Proposal (implementation ready, PR to follow):

1. portable_text element — value is a Portable Text block array; the dialog mounts the existing PortableTextEditor, so structure (blocks, spans, marks, markDefs, keys) round-trips intact and is never stringified. Runtime renderElement returns null like repeater.
2. block_list element — an ordered list of registered plugin blocks ({ _type, _key, ...fields } items). Dialog UX: add from the registered catalog, edit via the chosen block's own Block Kit form, duplicate, delete, drag reorder, and "Move to…" a sibling list when a form holds several lists (including lists nested in repeater rows). allowed_types, min_items, max_items bound it.
3. Duplicate on plugin block cards, inserting an identical copy after the original.

Both new elements are additive (new Element union members + validation), are accepted as repeater sub-fields, and change nothing for existing blocks. Motivation: CMS-native migration tooling needs imported rich text and nested layout content to stay fully editable in the stock admin editor without a private document format.