Add parameter and return types to ValidatorInterface::isValid()

[gsteel]

Q	A
BC Break	yes
QA	yes

Description

The main change here is changing the signature of ValidatorInterface::isValid() to public function isValid(mixed $value): bool;

The patch includes some other miscellaneous clean up:

• Add property types to common props such as $messageTemplates
• converts some protected static props to private constants

[froschdesign]

LGTM 👍🏻