AX API exposure of mprescripts and none elements #12
Description
Technically, <mprescripts> and <none> elements are only some kind of marker children of <mmultiscripts> to respectively separate post/pre scripts, and to indicate absent scripts. MathML Core just says to layout them as <mrow>. In the most common cases, they are just empty elements.
See https://w3c.github.io/mathml-core/#dfn-mmultiscripts
I wonder if for consistency and simplicity we could just use the mapping suggested in #11. But AX API mapping in the spec is currently "Not mapped" for <mprescripts> and "TBD" for <none>. My guess is that it does not matter whether <mprescripts> is exposed and how since assistive technologies will rely on AXMathPostscripts/AXMathPrescripts attributes anyway. However, <none> should show up in the accessibility tree, otherwise they can't be in the arrays returned by the previously mentioned attributes and one can't rely on parity of the index in the array to determine whether an element is a sub or sup script. I haven't checked how browsers currently expose them.