implement MathML

[prepare]

https://www.w3.org/TR/MathML3/

and LayoutFarm/Typography#20

see also https://www.iso.org/standard/58439.html

[zhhbo]

is it support mathml now?

[prepare]

Not now :)

[prepare]

see @brezza92's MathML in work-in-progress => LayoutFarm/Typography@8681780