Strengthen "$schema" + "$vocabulary" requirements

[handrews]

There is currently no normative language requiring "$schema" to be respected. Particularly for supporting optional vocabularies without requiring custom code (see #1300), there needs to be a mandatory requirement to inspect the meta-schema for $vocabulary. Guidance would be needed for the case when the meta-schema is not available. Probably also guidance that schema publishers are expected to make their meta-schemas as available as their schemas whenever they use custom meta-schemas. We probably also need wording around when the meta-schema is not in $schema but is provided as a media type parameter.

[handrews]

This issue doesn't mean that we have to keep $schema and $vocabulary exactly as they are, just that whatever we end up with in the next release needs normative language that guarantees predictable behavior.

[nilaygit-10721]

can you assign me this issue?

[gregsdennis]

Thanks for highlighting this issue. I need to recategorize it. Looks like it's already categorized properly.

Vocabs have been extracted into a formal proposal now. We have a lot of design work before we can pull the feature back into the spec. There's not much to do as far as writing at the moment.