feat(math): Support mathvariant on special extra greek characters

[Omikhleia]

On greek uppercase, the theta variant and the nabla:

On greek lowercase,

(I double-checked the codepoints, but Libertinus Math seems indeed to lack the bold sans serif and bold sans serif italic for all these extra greek variants, but the "variant theta symbol")

It also "extends" the use of italic for these lowercase characters when used alone (as a one-character <mi>), 'cause one expects:

``\def{left}{} \def{right}{} \left( \sum_{j=1}^4 \epsilon^j \phij + \left( \sum{k=5}^8 \epsilon^k \right) \phi_5 \right)

• \sum_{i=2}^8 gi \left( \sum{j=1}^4 \epsilon^j \phij + \left( \sum{k=5}^8 \epsilon^k \right) \phi_5 \right)^i``

(Compare with (La)TeX https://tex.stackexchange.com/a/116269)