[Elements - Accessibility Pane]: Adding more intuitive keyboard navigation to Accessibility tree
[Elements - Accessibility Pane]: Adding more intuitive keyboard navigation to Accessibility tree

Issue:
- Accessibility tree in Elements pane has unusual keyboard navigation

Before:
- Up Arrow -> select previous
- Down Arrow -> select next
- Left Arrow -> select previous
- Right Arrow -> select next
- Enter -> inspect node
GIF: https://imgur.com/NMOnRkW

After:
- Up Arrow -> select previous
- Down Arrow -> select next
- Left Arrow ->
	if on node with expanded children -> collapse children
	if on node with no expanded children -> select parent
- Right Arrow
	if on node with children -> inspect node (expands children)
	if on node without children -> nothing
- Enter -> inspect node
GIF: https://imgur.com/EP1B055

Bug: 963183
Change-Id: I9f862ba6a94980afbfbb0eaf807fd3b85d51d3e7
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2101371
Reviewed-by: Tony Ross <d5807c9a1621d46cc76cd6770054768aff0ae774@microsoft.com>
Reviewed-by: Kalon Hinds <0953a07de5dd99912b5643571c8ed9864c017a6d@microsoft.com>
Commit-Queue: Michael Liao <00d75d81082fdf74688ec453c516b42f7cbf0f40@microsoft.com>

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