Need a composable binding for BackButtonScreen

[rjrjr]

Missing from ScreenComposableFactoryFinder.

      // Support for Compose BackStackScreen, BodyAndOverlaysScreen treatments would go here,
      // if it were planned. See similar blocks in ScreenViewFactoryFinder
      ?: (rendering as? BackButtonScreen<*>)?.let {
        ScreenComposableFactory<BackButtonScreen<*>> { backButtonScreen ->
          // now do the back button stuff
          WorkflowRendering(backButtonScreen.content)
        } as ScreenComposableFactory<ScreenT>
      }