Describe a paragraph type declaratively

Problem/Motivation

A paragraph type is a bundle with fields, so most of them need exactly what any bundle needs: a structure. What makes them look special today is that the forks render them through a switch keyed by type name instead of through a structure.

Proposed resolution

• Generate a structure per paragraph type, the same way as for any other bundle, and render through it.
• Keep the type name in the output, since the front end switches on it.
• Nested paragraph fields resolve through the paragraphs shape, to the depth the scanner recorded.
• A type with no structure returns its type name and a marker rather than nothing, so a missing mapping is visible in the response and in the queue.

Remaining tasks

Everything.