Potentially part of #41379.

On occasions where patterns designed to be used for template headers / footers are inserted it might be good to automatically create template parts, or at least offer a way to do so. This helps in a couple of ways:

1. It unlocks functionality associated with semantic template parts (replacing)
2. Reduces the chance of using the same pattern in multiple templates without synchronisation

A very rough mockup:

To do:

• Convert patterns to template parts when inserted via the Inserter
• Convert pattern to template part during template part replacement flow (see: Template editing: convert semantic patterns to template parts upon insertion #41397 (comment))