Support prefixItems for tuple validation

[isti115]

From the JSON Schema 2020-12 Release Notes:

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.

For an example with more explanation:
https://json-schema.org/understanding-json-schema/reference/array.html#tuple-validation

As per #1 (which has a broken link by the way), here is the respective file from the official test suite:
https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/main/tests/draft2020-12/prefixItems.json

In case there is interest in resolving this issue, I would be willing to look into the code (I've already checked the relevant parts of parsing and generation) and try to provide a pull request for it.

[bcherny]

This is a pretty unfortunate change in the spec. It means that the meaning of items is dependent on the version of JSON-Schema you use. But since a JSON-Schema is not required to specify its version, this leaves the interpretation of items ambiguous in practice 🫤.

A middle of the road approach to support this without breaking others' code may be to expose the new behavior behind a switch, say --json-schema-version=2020-12. We would check the option in the normalizer, and normalize prefixItems to items and items to additionalItems if needed.

Want to give this a shot?

[TheoAnastasiadis]

I thought I'd have a go at it.

Added a CLI argument as "--usePrefixItems" which defaults to false, I did not think specifying JSON schema spec version would be helpful at this point since only this version would be supported.

The main addition is a new normalizer rule; renaming the schema keys.

schema.additionalItems = schema.items
schema.items = schema.prefixItems
delete schema.prefixItems

I created a simple unit test as well. I will let it sink in cause maybe more test cases might be needed. I had difficulty understanding the E2E testing setup, so I did not add any new E2E tests, maybe if somebody would very briefly walk me through I am going to do so. Tests are passing, for now.