Fix old draft-07 schema in 2020 schema

[tarjelavik]

Fix #12, #8

AI helped fix and write this issue... :-/

The problem: schema/core.json declares $schema: draft 2020-12, but the ContextStringOrArray definition used the draft-07 tuple form of items (an array of subschemas):

"items": [
    { "type": "string", "format": "uri" }
]

In draft 2020-12, items must be a single schema (object or boolean) and applies to all array elements; the tuple/positional form was renamed to prefixItems. That's why the metaschema rejected /definitions/ContextStringOrArray/anyOf/1/items with "want boolean or object, but got array."

Since the intent is "an array of context URI strings," I changed it to apply the subschema to every item:

{
                    "title": "Linked Art with Extensions",
                    "type": "array",
                    "items": {
                        "type": "string",
                        "format": "uri"
                    }
                }

[tarjelavik]

I did not remove the duplicate prop in core.json, "referres_to_by". It was part of the PR to "main repo.

Removed in latest commit