Option analog to math-style in unicode-math

[omasanori]

I've been trying Euler Math (OpenType version of AMS Euler, successor of Neo Euler) with OpTeX. A problem is, Euler typefaces have no italic glyphs. Variables, displayed in italic with typical math typefaces, are in upright with Euler typefaces.
unicode-math has the math-style option to manage which kind of glyphs should be italic or not, along with other knobs. (See section 5 in unicode-math user manual.) While I am unsure if we want all the options unicode-math provides in OpTeX, they may be worth a consideration imho.

[omasanori]

Well, I overlooked section 1.9 in the OpTeX math manual...

[olsak]

I added these options right now. See 8bf1720.

[omasanori]

Thank you so much!