Use `additionalProperties: false` in strict meta-schema to catch typos in schema rules

The JSON Schema Draft-07 meta-schema (i.e. the schema that validates the schema) does not set additionalProperties: false, likely to support extensions to the schema. This has the major downside that typos go unnoticed.

To give a few examples I've seen:

{"foo": {"bar": { ... }}}  # wrong
{"foo": {"type": "object", "properties": {"bar": { ... }}}} # correct

{"additional_properties": false} # wrong
{"additionalProperties": false} # correct

Validators like jsonschema in Python simply ignore typos in a schemas, since after all it's valid under the draft-07 meta-schema.

I think it would be wise to do a pass over the current schemas collected in schemastore and try to correct them here or upstream.

I've quickly put a script together that does exactly that for draft-07, and it reports the following problems. Some of them are actual issues with the schema, some of them are due to use of extensions -- I did not filter anything out.

Start with the linked validation script and the Draft-07 meta-schema, then review its reported results against the current SchemaStore schemas. Separate actual schema typos from intentional extensions, correct issues here where appropriate, and identify changes that belong upstream; done means the review has been completed and the relevant schemas or upstream issues are addressed.