Introduce dynamic text styles and various font modifiers

[stackotter]

This PR introduces dynamic text styles with a similar API to SwiftUI. It also expands the existing set of font-related view modifiers, and introduces a set of APIs for constructing new fonts from existing fonts.

The set of text styles supported by SwiftUI differs by platform. I've standardised SwiftCrossUI's text styles on the set of styles supported by SwiftUI on macOS. I've extrapolated values for any text styles that SwiftUI doesn't support on certain platforms.

I've based the font size and weight of each text style off Apple's typography guidelines. By default, the text styles are resolved by SwiftCrossUI and only depend on the backend's reported device class (e.g. desktop, tablet, etc). However, backends can override the text style resolution process if needed.

Example usage of new features

Text("Your Recipes")
    .font(.title.fontWeight(.bold))

Text("Recently Cooked")
    .font(.title2)

ForEach(recentlyCooked) { recipe in
    HStack {
        Text(recipe.name)
            .emphasized()
        Text(recipe.author)
            .fontWeight(.light)
    }

    Text("\(recipe.stars)/5")
        .fontDesign(.monospaced)

    // Alternative approach
    Text("\(recipe.stars)/5")
        .font(.body.monospaced())
}