Implement `eql?` predicate to json-schema `const` keyword conversion

[jsonb-uy]

Hi! This change would allow us to convert the eql? predicate to the const json-schema keyword which was available since the draft-06 of the spec.

References:

• https://json-schema.org/draft/2020-12/json-schema-validation.html#name-const
• https://json-schema.org/draft-06/json-schema-release-notes.html#additions-and-backwards-compatible-changes

Thank you! 🙇