Enable setting custom `TreeItemElement` component

[cunj123]

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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.