New writing style

[p-x9]

closes #13

Currently, to get the color, write as follows:

Color.iOS.systemBackground

The following new writing methods are also available in this pull request.

Color.iOS(.systemBackground)

The new writing style looks simpler in the following cases:

var body: some View {
    Text("hello")
        .foregroundColor(.iOS(.darkText)) // New
        .foregroundColor(Color.iOS.darkText) // Old
}

The previous way of writing is also available at the same time.
Therefore, there is no need to modify the existing used parts