Mapping of <none> and unknown MathML Core elements

[fred-wang]

cc @asurkov

In w3c/mathml-core#173 we are removing <none> from MathML Core.

Currently, the mapping for <none/> (empty scripts in <mmultiscripts>) seems incomplete. IIUC unknown MathML Core elements are not exposed in the accessibility tree, which means that we either need to keep mapping for <none/> in MathML AAM or add some generic mapping for unknown MathML elements.

[fred-wang]

Tentative patch in Chrome: https://chromium-review.googlesource.com/c/chromium/src/+/4061458
This is incomplete, but hopefully illustrates the issue.
One test for unknown elements is https://source.chromium.org/chromium/chromium/src/+/main:content/test/data/accessibility/mathml/unknown.html