…matic aggregation of children's a11y labels.

After consulting Apple documentation and working with some a11y experts,
we've found that aggregating objects that have a11y labels but are not
themselves interactable is significantly preferred for these users.

It makes it much quicker to navigate scrolling content if VoiceOver only
stops to select entire cells, and then allows drilling down into the cell
to select individual components. This implementation achieves that behavior.

We should consider enabling isAccessibilityContainer by default on ASCellNode.
This would be an improvement for 95% of a11y use cases. Aggregation can be
enabled or disabled on any node.