Expose index

[Ivo-Evans]

Relates #37 #40 #18

Tested on Svelte ^3.46.2 with a list of 1,533 items - it rendered OK, but if folks are troubled by the performance of iterating through the list beforehand, maybe there's some other solution. It could be an option that the user opts in to, for instance.

Use it like this

  <VirtualList items={statements} let:item let:absoluteIndex>
    <Statement {...item} index={absoluteIndex} />
  </VirtualList>