Add public _ModifierStack type

[carson-katri]

This adds a new type that can be used by add-on libraries that enables modifiers to be passed as an argument to another modifier.

For example, the chart_plot_style modifier in Swift Charts accepts modifiers as an argument:

chart_plot_style(
  aspect_ratio(1, content_mode: :fit) |> frame(height: 200)
)

See liveview-native/liveview-native-swiftui-charts#96 for the implementation of this modifier.

[supernintendo]

Looks good to me