Add @property $date to the DateTimeItem

Nice, the phpstan fail is because we are ignoring this error in the phpstan baseline and that is no longer necessary. That validates that adding this @property is useful not only downstream but in core also. Needs work to remove the ignore rule from the phpstan baseline.

Now MR looks good phpstan issue fix.

This fixes phpstan errors in \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem::onChange that were in the baseline.

I think we should update \Drupal\datetime\Plugin\Field\FieldFormatter\DateTimePlainFormatter::viewElements and other related formatters. It seems we're using the local variable $date simply so we can typehint it to DrupalDateTime. I think instead we should typehint $item to DateTimeItemInterface and drop the local variable.

Wondering though if DateTimeItem::$date could be null though, in which case we need a union type on the @property.

I'm wondering why we wouldn't protect the date property and add something like a getDate() method in that case (from a position of ignorance).