Add support for mathematical variation sequences

[MDLC01]

Unicode defines some variation sequences (when a character is followed by a specific "variation selector" codepoint, an alternative glyph should be used). Those variation sequences (not all of which are mathematical) are defined in StandardizedVariants.txt. In particular, there are standardized variation sequences for capital, non-bold, calligraphic letters: https://www.unicode.org/versions/Unicode16.0.0/core-spec/chapter-22/#G82733.

Supporting those variation sequences would be a great long-term goal. Of course, supporting more bare mathematical symbols is higher priority, but I am creating this issue to bring variation sequences to your attention.