Fix 'Access to an undefined property' PHPStan L0 errors - public properties

Not sure about returning $info array to extension class which was just removed and ongoing work happening to fix it with getters/setters #3292759: Create getters and setters for dynamic Extension properties

$info has been a public property on the extension object for a very long time. This change is only documenting that fact. It's not been removed.

Committed and pushed f76cebd189 to 10.1.x and 7fe5869f20 to 10.0.x. Thanks!

Missed this on my earlier review but this IMHO has some problematic parts.

This didn't just add new properties but also added types to existing properties by moving them to the base class, that broke the aggregator module:#3294389: Fix fails in HEAD and other modules could be affected as well in contrib. Isn't adding a type to existing properties a BC break? Especially in a base class.

Another strange part is that it defines the original property for _some_ entity types, but $entity->original is a generic entityconcept and this conflicts with #2839195: Add a method to access the original property which I finally resurrected. Sure, we need a BC layer for that anyway but that might complicate that and feels like a very half-baked solution with the only purpose of reducing the baseline.

This also fixed #3035618: Extension.php code quality: declare info array as public class field :)