Implement keyboard accessibility

[oliviertassinari]

The component doesn't have keyboard support. I think that it would be interesting to implement it. The accessibility guidelines can be found at https://www.w3.org/TR/wai-aria-practices/#TreeView.

Here is an example in the Chrome Dev Tools:

---

Material-UI used to rely on react-inspector in the Default Theme inspector but it was replaced so we could dogfood our implementation of the TreeView component. I wish we could go back to use react-inspector while still dogfooding our TreeView 😍(happy to work on it if needed).

[rvantonisse]

I second this. Looks like last missing piece for a basic accessible tree.

This is a link directly to the current WAI APG tree pattern: https://www.w3.org/WAI/ARIA/apg/patterns/treeview/