Allow `HorizontalScroll` HStack to be applied with custom modifiers

[PavelHolec]

To allow items that have a global effect (elevation etc), we need to be able to modify the HStack that is used inside the HorizontalScroll - or, to allow call site to provide this stack wrapper.

example:

HorizontalScroll(..) {
   ForEach(...) {
      item
   }
} { content in
   HStack(..) {
      content
   }
   .clipShape(..)
   .drawingGroup()
   .elevation(..)
}