fix(useFocusZone): handle add/remove nodes that have focusable children

[dgreif]

Within a focus zone, we watch for child nodes to be added/removed. Currently, we only do something with those changes if the node that was changed isFocusable, or has been registered as a focus target. If that condition is met, we search all children of the changed node to find focus targets. This setup does not account for scenarios when the added/removed node is not itself focusable, but it does have children that are focusable. This change removes the top-level check and simply always searches for focusable children. This is a noticeable issue in SelectPanel, where the list items are added/removed frequently with changes to the filter.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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Latest commit: 83484a8

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[T-Hugs]

Good fix! I think I had the isFocusable check in there before adding support for iterating the focusable children and didn't think to remove it.