feat(bottom-navigation): add placeholder prop

[YHoney7]

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

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Name	Status	Preview	Comments	Updated (UTC)
varlet	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 16, 2024 1:35pm

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[chouchouji]

还需要修改 types 的 对应文件

[YHoney7]

好的

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.75%. Comparing base (560af0c) to head (254e604). Report is 12 commits behind head on dev.

Files	Patch %	Lines
...rlet-ui/src/bottom-navigation/BottomNavigation.vue	83.33%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1618      +/-   ##
==========================================
- Coverage   92.84%   92.75%   -0.10%     
==========================================
  Files         306      306              
  Lines        7172     7193      +21     
  Branches     1785     1790       +5     
==========================================
+ Hits         6659     6672      +13     
- Misses        447      454       +7     
- Partials       66       67       +1     

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