Tree: include a list of filtered node keys on filter event

[erwanlfrt]

The Primevue tree component has a fantastic and complex filtering mechanism that can filter nodes either using lenient or strict algorithm. However, the filter event does not return the list of filtered nodes.
Since the filtering mechanism is quite complex, it's hard to know from the parent component which nodes are being filtered.
It would be great to attach the computed filteredValue to the filter event or at least a list of filtered node keys.