Warn on unsatisfiable schemas and other quality of life checks

[dtheodor]

jsonschema allows to shoot yourself in the foot by specifying unsatisfiable conditions, such as incompatible types for an attribute. This is in most cases a schema designer error. It would be nice if this library could emit warnings in such and various other design errors.

This project deals with this nicely https://ajv.js.org/strict-mode.html