Add a way to validate TLDs

Add a new function Pdp\Parser::getRawPublicSuffix() that can be used to check a host’s TLD for validity. The method returns `false` for invalid TLDs. Move most logic from Pdp\Parser::getPublicSuffix() into this new method. Ensure existing unit tests pass. Add unit tests for validating the one new logic rule introduced by the new method.