support for the next draft after 2020-12

[karenetheridge]

There is no label/date assigned yet, but we can implement support now under this name, for testing.

• contains now supports objects as well as arrays (and unevaluatedProperties behaviour changes as a consequence)
• most URIs are now IRIs
• propertyDependencies keyword
• $dynamicRef semantic changes
• requireAllExcept keyword

[karenetheridge]

see also json-schema-org/json-schema-spec#1148