#223 feat(config): ship JSON Schema for the configuration file

[arseniy-gl]

Add a JSON Schema for the openapi-format config file.

The config file has no schema today. Editors cannot help the user. A typo in a key name passes the CLI without error.

Changes:

• Add schemas/openapi-format.schema.json (JSON Schema 2020-12, 32 properties, 5 nested sets)
• Ship the schema with the npm package
• Add a README section with a $schema example
• Add a CHANGELOG entry under unreleased

How to use:

{
  "$schema": "https://openapi-format.com/schema/v1.json",
  "sort": true
}

Fixes #223.