Rename icrate features to end with `_core`

[madsmtm]

Rename Foundation to Foundation_core (and similarly for the other features).

[mwcampbell]

Would this be a precursor to being able to build icrate with fewer bindings for large frameworks like AppKit? The AppKit feature currently enables several bindings that AccessKit, for example, doesn't need.

[madsmtm]

No, the proper way to do that is to do cfgs based on file names instead.

I remember trying to make that work before, but it's quite difficult to do with Libclang, I'm hoping to get to it once we get further with https://github.com/madsmtm/objc2/issues/345.