Repeat blocks for each item of a multi-value field

Problem/Motivation

A binding gives one value. A multi-value field needs its design repeated for each item, like the Post Template block does for posts.

Steps to reproduce

Proposed resolution

• A "Field items" block repeats its inner blocks for each item of a field.
• Bindings in the inner blocks use the item from block context.
• In the editor, items are added and removed through the field widget.

Remaining tasks

User interface changes

The "Field items" block.

API changes

Data model changes