Move the REST API JSONP callback validation check into a separate function named `wp_check_jsonp_callback()`. This allows plugins to use the built-in validation when handling JSONP callbacks.
Extremely Important Note: If you send JSONP in your custom response, make sure you prefix the response with `/**/`. This will mitigate the Rosetta Flash exploit. You should also send the `X-Content-Type-Options:nosniff` header, or even better, use the REST API infrastructure.
Props rmccue.
Fixes#28523.
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