Incorrect handling of format-assertions

[big-andy-coates]

Hi @jimblackler,

Thought I'd let you know that I think this library is currently mishandling schemas that have format-assertion listed in its $vocabulary.

Turns out that having "https://json-schema.org/draft/2020-12/vocab/format-assertion": false e.g.:

"$vocabulary": {
        "https://json-schema.org/draft/2020-12/vocab/core": true,
        "https://json-schema.org/draft/2020-12/vocab/format-assertion": false
    },

...doesn't turn off format assertions, as I'd first assumed. Instead the false just means parsers/validators that don't understand format-assertsions can still parse the schema, (a value of true means such implementations should error out).

SchemaFriend fails 2 of the optional/format-assertions.json tests. I'm assuming this is because it turns off format assertions when it sees the "https://json-schema.org/draft/2020-12/vocab/format-assertion": false.

Here's the slack conversation about what the false means...