Add svelte sticky groups

[kbaratyants]

Summary

We needed sticky date-separator headers in a chat-style virtualized list (Svelte), similar to what's already possible in React/Vue/Solid via the "Sticky Group" pattern. The Svelte package didn't support this because Virtualizer/VList only accepted item as an HTML tag name — there was no way to attach extra style/class/attributes to a specific item's wrapper element, which is required to flip an item from position: absolute to position: sticky at scroll time.

Rather than hack around this at the story/consumer level, we added first-class itemProps support to the Svelte package (src/svelte), mirroring the existing Vue itemProps API:

• Virtualizer/VList now accept an itemProps?: (payload: { item, index }) => { style?, class?, ...attrs } prop.
• The returned style is merged on top of the item wrapper's base positioning styles (so overrides like position/top take effect), and any other returned attributes (e.g. class) are spread onto the wrapper element.
• WindowVirtualizer is untouched — Vue doesn't expose itemProps there either, so there's no parity gap to close.

With that in place, we ported the existing "Sticky Group" Storybook story (already present for React/Vue/Solid) to Svelte, demonstrating sticky group headers built entirely on public API — no internal hacks needed.

No dedicated unit test was added for itemProps, matching the existing convention in this repo (the Vue itemProps implementation has no dedicated unit test either) — coverage here is the existing VList/Virtualizer SSR spec suite plus manual verification via the new Storybook story.

Test plan

• npm run test -- --project=svelte passes (existing suite, unchanged)
• npm run check:svelte reports no new errors
• npm run storybook:svelte → "advanced/Sticky Group" story: scroll through the list and confirm the active group header (0, 100, 200, …) sticks to the top and hands off correctly at each boundary