Require only app-extension-safe APIs

[mattlarose]

tl;dr: Marks Rive library as safe, to prevent potential App Store rejection.

If an app extension (e.g., a widget) depends on Rive, when you build your app you'll be warned that you're Linking against a dylib which is not safe for use in application extensions. Apple may reject the app for this.

A library is safe for application extensions if APPLICATION_EXTENSION_API_ONLY = YES.

Right now, APPLICATION_EXTENSION_API_ONLY is unspecified in Rive, but Rive is only using APIs that are safe for app extensions. Since Rive is safe, I marked it as safe. If an unsafe API is ever used in the future, Xcode will generate errors. I'd guess that given the usual unsafe APIs, this seems unlikely ever to be a problem for Rive.

I didn't see some project file management setup, so I edited the .pbxproj directly.

[mattlarose]

silly fork branch mishap, reopened in #267