Support stem darkening for CFF driver and when autohinting

According to https://github.com/googlefonts/fontations/blob/cce82fb6911a1cc2f6009fddc9ffbc756f65a20f/skrifa/src/outline/cff/mod.rs#L516-L519, Skrifa doesn't support stem darkening. I wish it would one day, to make it suitable for [gamma correct text rendering](https://freetype.org/freetype2/docs/hinting/text-rendering-general.html). It would be nice to also support it in the autohinter, for TrueType and CFF. The TTF driver can't support it directly, because you'd have to mess with the output of bytecode hinting. I think someone from Adobe talked on the FreeType mailing list about stem darkening horizontally at least after TrueType bytecode is done, because most fonts are hinted only vertically.

Start with skrifa/src/outline/cff/mod.rs at lines 516-519 to understand the current lack of stem darkening, then read the linked FreeType text-rendering guidance. Define how support should apply to the CFF driver and autohinter for TrueType and CFF; done requires an agreed design and coverage for those requested paths.

Written by the indexing model from the issue text.