Add a better treeview to the debugger

[glennw]

The current treeview component is very basic. We could consider using a more advanced treeview component (some probably exist already in the vue ecosystem).

This is a placeholder - needs a bit more information to be actionable.

[glennw]

One specific thing is to support richer display of tree nodes.

For instance, we might like to have the serialized tree node structure contain a hash map of keys / values, and then each treeview has a template in the debugger that knows how to display a rich representation of that specific node (e.g. a clip-scroll node, or a render pass node).