MathML is coming, need to consider what we need to do to support it

MathML is new in Chrome 97, still behind a feature flag, but that means it's coming. Chrome is the last major browser without support for MathML, so native support for it is getting pretty close.

I doubt anyone's considered what impact native MathML has to axe-core rules, so we're going to need to look into that in the near future. First priority there is to make sure we don't produce false positives. Second is to figure out what new rules we can write for it.