Expose virtual index

{{#virtual-each
  height=200 // required: total height
  itemHeight=36 // required: row height
  onBotttom=(action 'handlePageBottom') // optional: invoked when the scroller hits the bottom
  positionIndex=0 // optional: used to scroll to a specific item index
  items=items as |item actualIndex virtualIndex|
}}
  {{actualIndex}} {{virtualIndex}} {{item}}
{{/virtual-each}}