Support Draft 2020-12 changes to items and additionalItems

[yhuard]

Hi!

Thanks for this great lib! I was playing around and noticed that it doesn't fully support JSON Schema Draft 2020-12.

More specifically, do you plan to support the changes introduced in Draft 2020-12 regarding the tuples validation?

The items and additionalItems keywords have been replaced with prefixItems and items where prefixItems has the same functionality as the array-of-schemas for of the old items and the new items keyword has the same functionality as the old additionalItems keyword.

https://json-schema.org/draft/2020-12/release-notes#changes-to-items-and-additionalitems