Support "prefixItems", changes to "items"

[charjr]

OAS 3.1 adds prefixItems, it changes items.

prefixItems takes a list of schemas. The index of the schema corresponds to the index of the item in the array that it will validate. i.e. the first schema validates the first item.

items now takes a bool or a schema, this dictates validation of any items unspecified in prefixItems.

Useful links:
https://json-schema.org/understanding-json-schema/reference/array.html#tuple-validation
https://json-schema.org/understanding-json-schema/reference/array.html#additional-items