Warn deprecated property

[eggplants]

deprecated: https://www.learnjsonschema.com/2020-12/meta-data/deprecated/

It seems like displaying warnings for properties marked as deprecated would be a good idea.

[eggplants]

Since json-schema-library does not treat deprecated fields as errors, it seems we'll need to implement our own detection.

[eggplants]

They added deprecated-warning in sagold/json-schema-library@6f0416d.