Fix "license" field in composer.json + add license headers to the entry PHP-file

[versusbassz]

Links

• https://getcomposer.org/doc/04-schema.md#license
  • https://spdx.org/licenses/

Also:

• https://developer.wordpress.org/plugins/plugin-basics/header-requirements/
• https://developer.wordpress.org/plugins/wordpress-org/detailed-plugin-guidelines/ (section 1)