New polyfill for malignmark/maligngroup

I had Opus 4.8 create a polyfill for malignmark/maligngroup as defined in
MathML 3. I thought it would be large, but it turned out to be ~600 lines
of Javascript and pretty much worked the first time. Perhaps the algorithm
in the spec helped out and perhaps my suggestions that it measure things to
get the alignment right helped (a trick a number of the other polyfills
use). I have tested it on Chrome and Firefox.

There are a few examples here
<https://w3c.github.io/mathml-polyfills/malign/index.html> including the
tricky spec example and one that I created from an older version of MS Word
(the newer versions don't use malignmark/maligngroup).

It could really use more examples to make sure it fully works. Please
contribute some if you know of others.

    Neil