incorrect use of mathvariant=normal for single character identifiers

[xworld21]

LaTeXML uses a naive check to decide if mathvariant="normal" should be applied to single character identifiers: https://github.com/brucemiller/LaTeXML/blob/3875cd64faeb5bdc275d595cb6af772958a51080/lib/LaTeXML/Post/MathML.pm#L717

However, MathML Core prescribes that the automatic italic conversion only applies to the characters in Appendix C.1, not all single character identifiers: https://www.w3.org/TR/mathml-core/#math-auto-transform

I don't think this results in an actual bug, because an extra mathvariant="normal" would simply be redundant, while having $variant eq 'italic' will probably have no effect. I just wanted to add a note about it.