AX mapping of <mfenced> element

[fred-wang]

I'm opening this for the record, but note that this element is not part of MathML Core.

It is still define in MathML 4: https://w3c.github.io/mathml/#presm_mfenced ; it's equivalent to an expanded form with <mrow>s and <mo>s (with separator/fence attributes).

WebKit maps that element to role/subrole AXGroup/AXMathFenced its open/close attributes to AXMathFencedOpen/AXMathFencedClose. The anonymous <mo>'s of the expanded form show up in the tree and have special subroles defined in #15