feat(app-bar): add placeholder prop

[YHoney7]

Checklist

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• Fix linting errors
• Tests have been added / updated (or snapshots)

Change information

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Issues

close #1610

Related Links

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Name	Status	Preview	Comments	Updated (UTC)
varlet	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 31, 2024 3:20pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (8ff21b9) to head (d9781ad).
Report is 10 commits behind head on dev.

❗ Current head d9781ad differs from pull request most recent head ccf3824

Please upload reports for the commit ccf3824 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1643      +/-   ##
==========================================
+ Coverage   92.78%   92.79%   +0.01%     
==========================================
  Files         307      307              
  Lines        7203     7213      +10     
  Branches     1794     1796       +2     
==========================================
+ Hits         6683     6693      +10     
  Misses        453      453              
  Partials       67       67              

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