Import SwiftGen package

[PaulRWillis]

Importing package ready for modularizing iOS assets including images and colors.

Next steps:

1. Make separate UIColors module.
2. Add SwiftGen stencil to UIColors and migrate across test color.
3. Migrate all colors to UIColors under /Resources/Colors.xcassetcatalog.
4. Clear up legacy string-based color accessors (Color extension)
5. Make separate UIImages module.
6. Add SwiftGen stencil to UIImages and migrate across test image.
7. Migrate all images to UIImages under /Resources/Images.xcassetcatalog.
8. Clear up legacy string-based image accessors.

Note that the app's "app icons" are still in the asset catalog instead of separate images from Icon Composer. We might need to keep an asset catalog of app icons in the main project and out of SPM to support this for now.