IBM Plex Math: Support for standardized mathematical variation sequences

[Enivex]

https://www.unicode.org/Public/UCD/latest/ucd/StandardizedVariants.txt

In particular, the use of U+FE00 and U+FE01 to choose between the roundhand and chancery styles of calligraphic letters is important, because those can be semantically different. (\mathcal vs \mathscr)