skip CLI should have a `skip font` command

Apropos #345 and #346, it's very complicated to set up fonts correctly in a Skip app, and worst part is that when you mess it up, the build succeeds, and the app just silently shows the system-default font, with no warning or message in the build/runtime logs about missing fonts.

The skip CLI has a skip icon command that takes care of icons for you; I think it would be really helpful to have a skip font command that can take care of fonts, too.

You'd pass it the name of a file, like skip font ProtestGuerrilla-Regular.ttf, and it would:

• Read the Postscript metadata and compute a font file name to match Android standards (protest_guerilla.ttf)
• Copy the file into Sources/MyApp/Resources with the right name
• Add any project references / symlinks necessary to make the font work in Android
• Add the file to UIAppFonts
• Print out example code of how to use the font, ensuring that the developer uses the font's PostScript name and not the file's name.

Text("Hello, world").font(.custom("Protest Guerilla", size: 30.0))

Maybe even generate an extension to Font that would let you use the font in a typesafe way.

public enum BundledFont: String {
    case ProtestGuerrilla = "Protest Guerrilla"
}

extension Font {
    public static func bundled(_ font: BundledFont, size: CGFloat) -> Font {
        return .custom(font.rawValue, size: size)
    }
}

Then, skip font could suggest:

Text("Hello, world").font(.bundled(.ProtestGuerilla, size: 30.0))

Start with the existing skip icon CLI command and the font resource handling described in the issue. Define how metadata-based naming, Android references, UIAppFonts, usage examples, and optional type-safe APIs should fit together; done means skip font handles the requested file and produces usable guidance for both platforms.