Update VirtualScroll.svelte - expose "index" to item

[nickgraffis]

Previously you could not do this:

<VirtualScroll let:data let:index> ... </VirtualScroll>

now you can.

solves for #8

[v1ack]

Can you up the version in the package.json and mention it in readme? After that I'll merge

[nickgraffis]

Of course, thanks for your hard work! 🙏