Display MathML when MathPlayer is installed

When MathPlayer is installed, we show the MathML instead of the SVG fallback.
This will in particular allow the sync highlighting feature of MathPlayer.
This also introduces a module ext.math.js that could be used to do some
Javascript postprocessing.

Bug: 71748
Change-Id: I438a20032c312d12321ca4c5686bcfd107656b37