As proposed by @felixarntz in #443, this check would help encourage user to switch to non blocking enqueues.

Potentially we could also accept a blocking script as long as it's $in_footer. But it would definitely warn about a blocking in head script.

@swissspidy pointed out that The corresponding PHPCS sniff does not yet support the new $args param we added in 6.3. It was also indicated that the sniff might actually be removed because it's harder to detect. So probably needs to be a runtime check.