expand and collapse of tree through keyboard events

[ramcharan]

For the Tree component in prime react, I had an issue with keyboard strokes events, the expand and collapse arrow should get focus through the keyboard navigation.
On inspecting the tree element, the tabindex=-1 for element toggler button(tree expand and collapse button)

<button type="button" class="p-tree-toggler p-link" tabindex="-1"><span class="p-tree-toggler-icon pi pi-fw pi-chevron-right"></span></button>.

please do update tabindex=0 for the toggler icon, so that focus can be achievable.