Upgrade for ajv v8
Support JSON Schema draft-2020-12

Support JSON Type Definition schemas with --spec=jtd option.
Support plugins (e.g. user-defined keywords) in typescript (requires ts-node to be installed).

Do not change definitions to $defs when migrating schema to draft-2019-09 (#112)

Fix: pass $data option to ajv instance when --data option is used

See v4.0.0-rc.0

Use ajv v7
--spec= option to choose JSON schema specification:

• draft7: JSON Schema draft-07 (default)
• draft2019: JSON Schema draft-2019-09

Support schema migration to JSON Schema draft-07 and draft-2019-08 (defined by --spec= option).
Compile multiple schemas to a single module with multiple exported standalone validation functions.
Output standalone validation code to stdout (-o without parameter) for formatting with js-beautify or another utility.

Allow strict-mode options - --strict-keywords, --strict-defaults, --strict-numbers (@Agh42)

Support json5 and yaml file types (@G-Rath).
Update dependencies and tested node versions.

Support --data option (passed as $data to Ajv).
Support --add-used-schema option (passed as addUsedSchema to Ajv) (@Relequestual)

Update to ajv v6.0.0