Move NodeInterface::PUBLISHED and NodeInterface::NOT_PUBLISHED constants to EntityPublishedInterface

As their names suggest those constants should belong to EntityPublishedInterface. Basically any entity type that implements EntityPublishedInterface needs those constants.

Another issue here is that EntityPublishedTrait::isPublished() does not respect those constants. That could be fixed like follows.

- return (bool) $this->getEntityKey('published');
+ return $this->getEntityKey('published') === self::PUBLISHED;