refactor: opt getSize speed

[zombieJ]

优化 getSize 速度，重复读取时会复用先前生成的缓存。

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
virtual-list	❌ Failed (Inspect)			Aug 18, 2023 8:31am

[codecov]

Codecov Report

Merging #222 (77806ed) into master (cb8d647) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 77806ed differs from pull request most recent head bfccaf1. Consider uploading reports for the commit bfccaf1 to get more accurate results

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   99.48%   99.49%   +0.01%     
==========================================
  Files          16       17       +1     
  Lines         585      597      +12     
  Branches      121      125       +4     
==========================================
+ Hits          582      594      +12     
  Misses          3        3              

Files Changed	Coverage Δ
src/List.tsx	99.46% <100.00%> (-0.04%)	⬇️
src/hooks/useGetSize.ts	100.00% <100.00%> (ø)
src/utils/CacheMap.ts	100.00% <100.00%> (ø)

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