refactor: replace scroll logic with inner width

[zombieJ]

• Filler 设置宽度为 scrollWidth 时会导致 sticky: right 无效。把它挪到里面的 Item 上做 margin 偏移以支持 sticky。
• 支持 extraRender 将额外内容添加至滚动容器中。

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Name	Status	Preview	Comments	Updated (UTC)
virtual-list	❌ Failed (Inspect)			Aug 17, 2023 2:45am

[codecov]

Codecov Report

Merging #212 (3309564) into master (db4f5c9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 3309564 differs from pull request most recent head 6153e75. Consider uploading reports for the commit 6153e75 to get more accurate results

@@           Coverage Diff           @@
##           master     #212   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          16       16           
  Lines         543      544    +1     
  Branches      108      108           
=======================================
+ Hits          541      542    +1     
  Misses          2        2           

Files Changed	Coverage Δ
src/Filler.tsx	100.00% <100.00%> (ø)
src/List.tsx	100.00% <100.00%> (ø)
src/hooks/useChildren.tsx	100.00% <100.00%> (ø)

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