Expose "math" object attribute in ATK/IA2 for math

[aleventhal]

Currently JAWS and NVDA support math on ROLE_SYSTEM_EQUATION objects by using ISimpleDOMNode::get_innerHtml().
In the interests of deprecating and removing ISimpleDOMNode (for performance/memory reasons), browsers should also expose math on these objects via the object attribute "math", and it should contain the innerHTML of the math subtree.

This mapping is relevant for both MathML subtree roots and role="math".

Chromium has added support for this in recent Canary builds.
Mozilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1924832
NVDA issue: nvaccess/nvda#17296