add support for mathalfa.sty

[xworld21]

As reported by @rzach in vlmantova/bookml#11, when using \usepackage[cal=boondoxo]{mathalfa}, LaTeXML should redefine \mathcal as an alias for \mathscr. As of now, it annotates the output of \mathcal with the class ltx_math_caligraphic and so BookML ends up using the wrong font.

This will become marginally more serious if you merge #2244, as the characters themselves will get the wrong Unicode variation selector, and so browsers and MathJax will show the wrong glyphs (if and when UVSs get implemented more widely).

I suppose there may be other packages where \mathcal is actually roundhand and need a similar treatment.

[xworld21]

(Actually there is no binding for mathalfa, but it seems easy enough to implement.)