Automate creating the OAS 3.0.x compliance schema

[handrews]

• use alterschema to convert the OAS 3.0 schema to draft 2020-12
• possibly use a JSON Patch to fix oddities (e.g. we don't need "^\$ref$" as a patternProperty, we can just make "$ref" a property)
• use a JSON Patch document to add all custom keywords (and update the $id)
• if necessary, have a meta-schema with the custom vocabularies and patch $schema accordingly

Also: need to take into account that OAS 3.1's schema is across multiple files, but does not need alterschema migration. It may make sense to factor out a common JSON Patch file, so document the possibility in case adding 3.1 support does not fit into the timeframe.

[handrews]

Completed as of commit d8c268a4798c9cf3ec77968fc088ec2c4521a934.

OAS 3.1 and multi-file schema support deferred to #6. See also #5.