Add new CI check for appropriate generic typehints

[GuySartorelli]

silverstripe/silverstripe-framework#11066 introduced generic typehints in a bunch of places. We should have a CI check to ensure code changes don't deteriorate this level of type hinting.

Related

• Validate @method docblock tag in CI #171

Notes

• While it would be awesome to have CI check that the correct type is being hinted, the most important part is that a type is hinted. We don't have any CI checking for correct types right now, so adding that is out of scope for this card unless it's trivial.

Acceptance criteria

• CI rules are added using a standardised tool (e.g. PHPStan or PHPCodeSniffer)
• All subclasses of ContentController, Extension, and implementations of SS_List have a @extends PHPDoc annotation if the class they're a direct subclass of uses a @template annotation
• If the type in the @extends annotation is a generic type, there is a corresponding @template annotation
• Methods which return an implementation of SS_List include a generic hint (e.g. instead of just @return DataList it should be @return DataList<SomeDataModel>
• Classes mentioned in annotations must either be in the same namespace as the class referencing them, or have a use statement