Object and enum types should be defined in $defs

[jack-berg]

More cleanup as we approach stability.

Here I ensure the schema consistently uses JSON schema's schema / subschema options with a new schema modeling rule:

In order to promote stylistic consistency and allow for reuse of concepts, object and enum types should be defined in either as a top level schema document or as a subschema in a schema document's $defs.

The stylistic consistency part is obvious.

"allow for reuse of concepts" is less obvious. When a type is defined in $defs we can reference as many times as needed in properties. When a type is defined directly in a property (or inline if you prefer), that type can only describe the requirements for that single property.

This guidance applies to object and enum types. This means that excluded are all primitives and arrays.

[marcalff]

LGTM, see minor changes.