Scroll Driven Animation props

[argyleink]

:root {
  --entering-scrollport-1: enter 0% enter 25%;
  --entering-scrollport-2: enter 0% enter 50%;
  --entering-scrollport-3 enter 0% enter 75%;
  /* etc */
}

@bramus and I were chatting about how we can stash these magical combos into named props, help folks kickoff and get started!