Add grouped mode support

[mariia-mitiureva]

Is your feature request related to a problem? Please describe.
Hi, it's me again :) It would be great to have some component that behaves like the storybook Sticky example. But in this example virtual list item is the whole group, which means that I cannot scroll to any item inside a group using scrollToIndex. Also if there are many items in one group, it will break optimisation - all group items will be rendered.

Describe the solution you'd like
For references: virtuoso grouped mode, how Slack groups messages by date -
https://github.com/inokawa/virtua/assets/37265338/e39c7385-9183-42a3-86cd-96e8f2415b3b

Describe alternatives you've considered
I was thinking if it's possible to implement similar behavior on my side using VList, but no solution yet

[inokawa]

Yes, sticky item feature was asked several times, and there are no good way to do it without fork for now.

It will be implemented as a feature like rangeExtractor(#278 (comment)), VListHandle.getItemOffset(#278 (comment)) or else, but not yet decided.

[morajabi]

Will this support 2 levels of nesting of sections? eg. I'd like to group messages per day and per sender inside each day. In order to use native browser position: sticky; for showing sticky elements in each section.

[inokawa]

For now, what I'm planning for the first release is to add a functionality to keep items offscreen at any indexes.
With position: sticky and some styling by user, it will work as sticky header of grouped items.

I'm not familiar to nested sticky styling. I think it depends on markup but perhaps it will work.

[inokawa]

I recreated sticky example based on keepMounted feature added in #429 .
https://inokawa.github.io/virtua/?path=/story/advanced-sticky-group--default
https://github.com/inokawa/virtua/blob/main/stories/react/advanced/Sticky%20Group.stories.tsx

Closing this issue because it will solve the problem originally raised. It will support GroupedVirtuoso-like "global stickiness" but I'm not sure for "ranged stickiness"(explained in #278).
Thank you!