Use keyboard to expand the tree

Patch attached.

I should specify that I'm not completely sure my approach is the right one. I'm absolutely open to any and all feedback, improvements, etc.

New version of the patch attached. This version corrects an issue where the behavior triggered twice when the spacebar was pressed, but only once when the enter or mouse buttons were pressed.

It might also be helpful to add the "role" attribute as button since these are also essentially buttons.

This should probably be a merge request now so changing to Needs work.

I added ✨ Screen Reader Accessibility Active as a related issue.

@mradcliffe I finally had a chance to circle back to this to open the merge request. I went ahead and added the role attribute as suggested too.