Feature request: expandable arrays

Like expandable tables in I7. Define an array, then later define more values for the array.

This would be handy for some extension use cases. Thread: https://intfiction.org/t/i6-suggestion-functionality-like-extend-but-for-arrays/63508

This would be a lot of work for questionable benefit, so I don't think it's a good choice. But I'm recording it here anyway.
