* Reimplement DNSCheckValidation to utilise get_dns_record and extend support for detection of Null MX records (RFC7505) and reserved, mDNS and private namespaces (RFC2606 & RFC6762)
* Change valid email tests to use github.com instead of example.com as example.com now (correctly) fails the tests due to the Null MX case
* Tweak to pass CI psalm issue
* Encapsulate dns check code. Simplify local and reserved domain checks. Switch test domain
* Set PHPDoc return type
* Use strict comparisons