Handle variant forms for pseudo-scripts better. (mathjax/MathJax#2519)

[dpvc]

This PR improves the handling of pseudo scripts so that both the superscript version (using msup) and the non-superscript versions will render properly (with a font that has the proper glyphs available, like STIX2). In the past, MathJax used the larger sized primes in the normal mathvariant, and this lead to incorrect sizes being used when the primes were not in msup. This PR now expects the regular size to be in the normal variant and the larger size in the -tex-variant font, and the mo element sets the correct variant for the position it is used in. It also remaps common characters that might be used for primes to their appropriate prime character (e.g., ' maps to U+2032 (prime)).

That means <msup><mi>x</mi><mo>'</mo></msup>, <mi>x</mi><mo>'</mo> and <mtext>x&#x2032</mtext> will all render properly (when the font contains the right characters, which the MathJax TeX fonts don't, but the STIX2 fonts do).

This is intended as a step toward making MathJax work better with STIX2, as well as setting the stage for modifications to the MathJax fonts later this summer.

[zorkow]

I remembered the discussion we had on that:
We wanted to handle pseudoprimes in core and giving the resulting node a pseudoprime property instead.
Firstly, we could expose the property (e.g., for SRE). And secondly, it would not need to be handled explicitly in the Wrapper.

[dpvc]

Right you are! Thanks for coming up with it again. It had totally slipped my mind.

[dpvc]

OK, I've moved the pseudo-script and prime checking to the MmlMo class and marked them with properties. The pseudo-prime is exposed on the serialized MathML output for your convenience in SRE.

[zorkow]

Only some minor things.