add client-side libraries using aria-owns

[pkra]

Cf. https://github.com/krautzource/mathjax-sre-walker/issues/13#issuecomment-538769599

Add the following:

• abstractTree.mjs - essentially the same tree structure as current tree.mjs but generated from presumed aria-owns structure
• treewalker.mjs - wrapper around navigator and abstractTree.