Enable setting custom `TreeItemElement` component

[cunj123]

Is your feature request related to a problem? Please describe.
I am trying to add animation to the item children list when expanding/collapsing. The easiest way to do that would be to put the transition on the height property and toggle the height between some value and 0.

Currently that is not possible becuase TreeItemElement doesn't render anything if the node isn't expanded.

Describe the solution you'd like
Allow developers to define a custom TreeItemElement where the hide/show behavior can me implemented.

Describe alternatives you've considered
Make the default behavior for showing/hiding nodes the one I described, but I don't think that's a good solution.

[lukasbach]

Hi, thanks for the suggestion. It's not really feasible to make the entirety of TreeItemElement customizable, but I extracted the logic for when it does render its items and when not customizable through the shouldRenderChildren prop in v2.3.5. That should hopefully help you achieve what you want.

See Demo
and Demo Implementation for details