Index is non-reactive (@next)

In the `next` branch, the index passed to the slot is from the non-reactive field:

https://github.com/Akryum/vue-virtual-scroller/blob/a6e07da79af0165a62fc6fc9bfb8ca08631b25ef/packages/vue-virtual-scroller/src/components/RecycleScroller.vue#L37

I'm seeing that my index-based view widget shows stale element, and I'm assuming that this is caused by this non-reactiveness.

Can you confirm that it can cause updates to be skipped for any index-based computed/templates, and if that's the case, make it properly reactive?