Add support for draft-2019-09

[jdesrosiers]

We need to add support for the following keywords to support draft-2019-09.

items - https://json-schema.org/keyword/draft-04/items
additionalItems - https://json-schema.org/keyword/draft-04/additionalItems

[jdesrosiers]

@Adityakumar37 There are only two keywords that you need to implement for this one, so this is a better place to start. As a first step, do some reading to make sure you understand what these keywords do and which draft-2020-12 keywords they are similar to. Respond here telling me what you learned.