Move Setting, Weight, Stretch, and Style types to font-types

[nicoburns]

Once font-types hits 1.0, these will be super-useful as interop types for the Rust crates ecosystem.

We may also wish to consider some renames:

• Weight -> FontWeight
• Style -> FontStyle
• Stretch -> FontStretch (or possibly FontWidth - matching CSS)

[rsheeter]

Seems reasonable at a glance, ty for sending. I will ping to see if anyone forsees problems.

[cmyr]

@nicoburns do you have a concrete use case/motivation for these changes? In general we want to keep changes in font-types minimal (specifically because it uses some unsafe, which means we potentially have to go through security review when making changes) and we'd be wary of scope creep (like wanting more CSS-style attributes.)

Put another way, would moving this API to font-types be useful to you, even if we were to never significantly add to the existing API?