Sans serif italic regular math greek support

[Firestar-Reimu]

latex3/unicode-math#620

I use sans serif to type matrices (and bold italic for vectors), but unicode-math does not support sans-serif for math greek, which is impossible to type like \mathsf{\eta X \eta}.

For XITS, you can use ss02 to load the sans-serif version.

But STIX2 does not provide sans-serif version of math greek.