fix(comp:button): fix icon not vertically centered

[liuzaijiang]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added/updated or not needed
• Docs and demo have been added/updated or not needed

What is the current behavior?

按钮中的图标没办法居中对齐

What is the new behavior?

使用flex居中对齐

Other information

此修改会影响到：带图标和没带图标按钮同时存在时的垂直对齐，请使用IxButtonGroup 或者 IxSpace ，或者flex样式包裹即可

[idux-bot]

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[codecov]

Codecov Report

Merging #1542 (d130de9) into main (6cd6155) will not change coverage.
The diff coverage is n/a.

❗ Current head d130de9 differs from pull request most recent head a1c0e10. Consider uploading reports for the commit a1c0e10 to get more accurate results

@@           Coverage Diff           @@
##             main    #1542   +/-   ##
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  Coverage   92.74%   92.74%           
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  Files         330      330           
  Lines       30642    30642           
  Branches     3499     3499           
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  Hits        28420    28420           
  Misses       2222     2222           

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