feat(app-bar): add placeholder prop

[YHoney7]

Checklist

List of tasks you have already done and plan to do.

• [ ] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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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