subvisual

[jkrumbiegel]

This PR introduces the subvisual function that can be used to style layers only when they match a target argument. For example, linear now uses a line and a band layer which can be identified via new :prediction and :ci labels. This allows to decouple line and band color, for example, or to create separate legends for both components:

mapping(1:20, 2 .* randn(20) .+ (1:20)) * (
    visual(Scatter) +
    linear() *
        subvisual(:prediction, color = :tomato, label = "Prediction") *
        subvisual(:ci, color = :teal, label = "CI")
) |> draw