Support `JSON Schema 2020-12`

[horiuchi]

ref #511

JSON Schema 2019-09

https://json-schema.org/draft/2019-09/release-notes.html
"$schema": "https://json-schema.org/draft/2019-09/schema"

New Features

• $anchor: Replaces the #plain-name form of $id, with a different syntax and approach
• $id: Only URI-references without fragments are allowed; see $anchor for a replacement for plain-name fragments; all other fragments in $id had undefined behavior previously
• $recursiveAnchor and $recursiveRef: Used for extending recursive schemas such as meta-schemas
• $ref: Other keywords are now allowed alongside of it
• unevaluatedItems: Similar to additionalItems, but can “see” into subschemas and across references
• unevaluatedProperties: Similar to additionalProperties, but can “see” into subschemas and across references

New Properties

• $defs: renamed from definitions
• $vocabulary: Has effects only in meta-schemas
• dependentSchemas and dependentRequired: split from dependencies
• maxContains and minContains: Assertion for controlling how many times a subschema
• deprecated: Used to indicate that a field is deprecated in some application-specific manner
• const: It is similar to enum, but has only one value

JSON Schema 2020-12

https://json-schema.org/draft/2020-12/release-notes.html
"$schema": "https://json-schema.org/draft/2020-12"

New Features

• $dynamicRef and $dynamicAnchor: The $recursiveRef and $recursiveAnchor keywords were replaced by the more powerful $dynamicRef and $dynamicAnchor keywords.
• Support Embedded Schemas and Bundling

New Properties

• prefixItems: replace from items
• items: replace from additionalItems

[glen-84]

2019-09 adds support for const as well, I think.