fix: missing validation errors (#16)

Fixes #15.

fix: update constraint for justinrainbow/json-schema to allow 6.3

Fixes #18.

fix: php 8.4 deprecations (#17)

feat: make compatible with PHP8 (#14)

* feat: make compatible with PHP8

Update README.md

feat: errors as JSON objects

fix: improve error messages for JSON Schema validators

feat: surface first error message

feat: add a validator that validates a collection of validators

feat: allow specifying the JSON Schema check mode