feat: add scaleOffset prop, set the default scaleOffset to 0.5

[coldice8]

之前默认放大缩小的倍数是 1 倍，这样无论是鼠标滚轮操作或是点击按钮操作，放大缩小的时候感觉非常突兀。 这里增加 scaleOffset 属性，且默认设置为 0.5 倍，更加接近普通操作情况。

[vercel[bot]]

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Name	Status	Preview	Updated
image	✅ Ready (Inspect)	Visit Preview	Aug 25, 2022 at 6:44AM (UTC)

[codecov[bot]]

Codecov Report

Merging #125 (6ab78fa) into master (200d856) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #125   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          276       277    +1     
  Branches        83        86    +3     
=========================================
+ Hits           276       277    +1     

Impacted Files	Coverage Δ
src/Image.tsx	100.00% <ø> (ø)
src/Preview.tsx	100.00% <100.00%> (ø)

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[afc163]

https://github.com/react-component/image/releases/tag/v5.8.0