[TypeDeclarationDocblocks] Add NarrowArrayCollectionUnionReturnDocblockRector

[TomasVotruba]

Adds a docblock rule that rewrites a Type[]|ArrayCollection @return union into a generic collection type.

Before

/**
 * @return LeadEventLog[]|ArrayCollection
 */
public function getSuccessful(): ArrayCollection
{
    return $this->successful;
}

After

/**
 * @return ArrayCollection<int, LeadEventLog>
 */
public function getSuccessful(): ArrayCollection
{
    return $this->successful;
}

• Matches a 2-type @return union of an array shape (Type[]) and a collection identifier (Collection / ArrayCollection, in any order).
• Key type is assumed int.
• Registered in TypeDeclarationDocblocksLevel.
• Covered with fixtures (transform, reversed order, skip already-generic, skip non-collection union).