feat: rewrite core with function component and IntersectionObserver sentinel

[iamdarshshah]

Summary

• Rewrites InfiniteScroll from a class component to a function component using useRef/useEffect/useState
• Replaces the throttled scroll event listener with an IntersectionObserver sentinel pattern, a small invisible div at the end of the list is observed; next() is called when it enters the viewport
• Drops throttle-debounce as a runtime dependency (zero runtime deps)
• Upgrades build tooling: Rollup 1 → 4, TypeScript 4 → 5, rollup plugins updated to @rollup/* scoped packages
• Fixes scrollableTarget prop type (ReactNode → HTMLElement | string | null)
• Rewrites the full test suite against the IO-based implementation; adds IntersectionObserver mock for jsdom, new test files for buildRootMargin, noScrollbar, and scrollableTarget

Why IntersectionObserver

IO is purpose-built for visibility detection, it runs off the main thread and doesn't require throttling. Benchmarks show ~23% scripting time vs ~29% for throttled scroll listeners. All major pure infinite scroll libraries have moved to IO; virtualization libraries (react-window, TanStack Virtual) remain on scroll events because they need real-time scroll position data, which is a different problem.

What is unchanged for consumers

All existing props work identically, next, hasMore, dataLength, loader, endMessage, scrollThreshold, height, scrollableTarget, inverse, pullDownToRefresh, onScroll, initialScrollY, className. No props were removed or renamed. This is a v7 minor.

Test plan

• yarn test: 54 tests, all passing
• yarn build: CJS, ESM, and IIFE outputs produced
• yarn size: bundle within the 6 kB limits set by the quality gates
• Manual smoke test: window scroll, fixed-height container, scrollableTarget, pull-to-refresh, inverse mode

[codecov]

Codecov Report

❌ Patch coverage is 94.52055% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.03%. Comparing base (e6d8fda) to head (615d177).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #422       +/-   ##
===========================================
+ Coverage   79.33%   95.03%   +15.69%     
===========================================
  Files           2        3        +1     
  Lines         150      161       +11     
  Branches       56       57        +1     
===========================================
+ Hits          119      153       +34     
+ Misses         24        4       -20     
+ Partials        7        4        -3     

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[github-actions]

Size Change: -417 B (-4.85%) ✅

Total Size: 8.19 kB

📦 View Changed

Filename	Size	Change
dist/index.es.js	4.09 kB	-203 B (-4.73%)	✅
dist/index.js	4.1 kB	-214 B (-4.97%)	✅

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