`$schema` on non-document-root schemas

per [2020-12 § 8.1.1.](https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-01#name-the-schema-keyword), "The "$schema" keyword ... MAY be used in the root schema objects of embedded schema resources."

This is possible to support in JSI. Support may be added pending feedback on this issue.