"type"/"items" and "discriminator"/"*Of" adjacency requirements

[handrews]

In 3.0, "type": "array" requires an adjacent "items", and in all of 3.x, "discriminator" requires an adjacent "*Of".

This is probably easiest to support with a metaschema patch since in 3.0 the schema dialect can't be changed. "discriminator" can also double-check in the keyword implementation.

h/t @MikeRalphson for calling attention to the array type/items requirement.