[BUG] No way to have original index with <GroupedVirtuoso />

[georgeLacs]

There is no way to have original index when using

Reproduction
https://codesandbox.io/s/staging-flower-q9xljn?file=/App.js

To Reproduce
Steps to reproduce the behavior:

1. Go to Sandbox
2. On generated list, indexes are displayed just like a flatMap. However it is important to return original index.
3. Original index, on example, is expected to be like

- Group 2
  Virtual Index: 20 / Original Index: 0 / (Group 2)
  Virtual Index: 21 / Original Index: 1 / (Group 2)
  Virtual Index: 22 / Original Index: 2 / (Group 2)
  Virtual Index: 23 / Original Index: 3 / (Group 2)
  Virtual Index: 24 / Original Index: 4 / (Group 2)
  Virtual Index: 25 / Original Index: 5 / (Group 2)
  Virtual Index: 26 / Original Index: 6 / (Group 2)
  Virtual Index: 27 / Original Index: 7 / (Group 2)
  Virtual Index: 28 / Original Index: 8 / (Group 2)
  Virtual Index: 29 / Original Index: 9 / (Group 2)

Expected behavior

• Update itemContent props to return (index: number, primaryIndex: number, groupIndex: number, data: D, context: C) or something like that

Desktop (please complete the following information):

• react-virtuoso 4.3.11 (latest)

[gnapse]

I spent a few minutes after my first use of virtuoso trying to figure out why it was calling itemContent with indices that were out of range for my grouped data structure. Until I realized that it was a flat index, what's being passed. That's unintuitive. In a grouped data structure, I would assume that item indices are 0-based per group. I agree with what's reported in this issue.