Expand SystemImage protocol to load scalable system images by default

[Mark Pospesel]

Our SystemImage protocol should load scalable system images by default. We do that by loading the image with UIImage.SymbolConfiguration(textStyle: aTextStyle). This allows the image scale according to the specified text style when Dynamic Type changes. And it will respond to Bold Text as well (to use heavier strokes).

1. add a static var textStyle: UIFont.TextStyle? with default implementation = .body
2. add a static var configuration: UIImage.Configuration with default implementation = UIImage.SymbolConfiguration(textStyle: textStyle) when textStyle != nil (need to see what configuration to use when it's nil. Maybe nil or maybe some default configuration that does nothing)

Idea is the following:

1. By default system images will scale according to .body
2. It's simple to override and specify a different text style if you need it a bit bigger or smaller; e.g. callout or title
3. If needed you can also provide any configuration. e.g. in addition to textStyle you might also want to configure weight (e.g. ultralight or bold) to adjust the appearance of the symbol. It's also an opportunity to apply hierarchical or palette information.