Add scrolling support

Adds three new values for touch-action: `pan-x`, `pan-y`, and `scroll`.

`pan-[xy]` allow for a scrollable div to scroll in the specified axis, or emit PointerEvents in the other axis. The decision to emit events or scroll is made on the first movement.

Together, `pan-x pan-y`, a scrollable element will always scroll, never emitting events.

`scroll` is equivalent to `pan-x pan-y`, and is a non-standard addition for brevity.