Support for YAML 1.2 JSON Schema and Core Schema

[perillo]

These schemas are new in YAML 1.2, and finally(?) solve the non portable behavior of different implementations:

• https://yaml.org/spec/1.2.2/#102-json-schema
• https://yaml.org/spec/1.2.2/#103-core-schema

One issue with the Core Schema is that the authors keep adding alias to builtins , like ~ being the same as null.