Add registerFont to Apple platforms

[darronschall]

I was running into a SwiftUI issue where the .fontWeight modifier stopped working when I moved fonts out of my iOS project into and into my shared library via moko-resources.

For example:

extension Font {
    public static var body: Font {
        return Font(MR.fontsOpenSans().regular.uiFont(
            withSize: UIFont.preferredFont(forTextStyle: .body).pointSize
        ))
    }
}

Text("Example")
    .font(.body)
    .fontWeight(.bold) // My project also has `MR.fontsOpenSans().bold` but this wasn't switching to it

It took a little bit to track down the problem. After some research, I found CTFontManagerRegisterGraphicsFont and later CTFontManagerRegisterFontsForURL

This PR does a few things:

• Moves iOS / macOS common code to appleMain FontResource to maximize code sharing, and creates platform-specific FontResourceExt for UIFont / NSFont usage.
• Adds an NSErrorException throwable that wraps an NSError to make it easier for Swift to inspect the nested NSError thrown.
• Adds registerFont to the Apple FontResource, meant to be called during App startup. Once a font is registered, then .fontWeight works as expected again.
• Moves the fontRef to a lazy delegate instead of always creating it in the class initializer. With the new registerFont, there are scenarios in which it is not necessary to create this reference.
• Improves memory handling when creating the fontRef

To use registerFont(), enumerate the list of fonts and call it on each one during application startup. Mine looks like this:

public static func loadSharedFonts() {
    let fonts = [
        MR.fontsOpenSans().regular,
        MR.fontsOpenSans().italic,
        MR.fontsOpenSans().light,
        MR.fontsOpenSans().semiBold,
        MR.fontsOpenSans().bold,
        MR.fontsOpenSans().extraBold,
        MR.fontsOpenSansCondensed().regular,
        MR.fontsOpenSansCondensed().semiBold
    ]
    
    fonts.forEach { fontResource in
        do {
            try fontResource.registerFont()
        } catch {
            // Example error inspection; some errors are safe to ignore. Check the error code
            // against CTFontManagerError values per https://developer.apple.com/documentation/coretext/ctfontmanagererror
            if let wrappedError = error.wrappedNSError {
                print("wrappedError domain: \(wrappedError.domain)")
                print("wrappedError code: \(wrappedError.code)")
                print("wrappedError userInfo: \(wrappedError.userInfo)")
            }
        }
    }
}

[darronschall]

I'm sorry, I originally wrote this PR based off of master. I've since rebased off of develop and cleaned up the commit history to make it easier to review.