Add support for small capitals (`smcp`) in Libertinus Math

[MDLC01]

I sometimes use small capitals in math content. When I use Libertinus Math, I need to switch to Libertinus Serif for small capitals because Libertinus Math doesn't support the smcp OpenType feature. Ideally, Unicode would have separate codepoints for mathematical small capitals letters, but for now, I suggest supporting small capitals in Libertinus Math through the smcp OpenType feature, just like is currently the case for Libertinus Serif.