improve: change default placeholder behavior, add previewPrefixCls prop

[shaodahong]

Ref: ant-design/ant-design#26296 (comment)

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[codecov]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          135       137    +2     
  Branches        30        32    +2     
=========================================
+ Hits           135       137    +2     

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

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[zombieJ]

approved~