refactor(DropdownMenu): move dropdown item overlay to dropdown menu

[yuhengshen]

将 drop menu item 的 overlay 共享，挂载到 drop-menu 上。避免切换菜单项时，屏幕闪烁。

修改前：

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.64%. Comparing base (ec5b45b) to head (9a93771).
Report is 65 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/vant/src/dropdown-menu/DropdownMenu.tsx	70.58%	0 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13411      +/-   ##
==========================================
+ Coverage   89.60%   89.64%   +0.03%     
==========================================
  Files         257      257              
  Lines        7013     7038      +25     
  Branches     1736     1745       +9     
==========================================
+ Hits         6284     6309      +25     
  Misses        384      384              
  Partials      345      345              

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[yuhengshen]

/run-ci