feat(DatePicker): support confirm and getSelectedDate method

[bluesky335]

issue：#12711
为 DatePicker 组件添加了 confirm 方法
为 DatePicker 组件添加了 getSelectedDate 方法

[chouchouji]

The docs has no return value, but the confirm method has.

[chenjiahan]

As far as I know confirm method has no return value..

[chouchouji]

The type of getSelectedDate can be inferred by ts, maybe no need to declare the return type.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.80%. Comparing base (103b0ba) to head (e51dd95).
Report is 4 commits behind head on main.

❗ Current head e51dd95 differs from pull request most recent head 9ca2054. Consider uploading reports for the commit 9ca2054 to get more accurate results

Files	Patch %	Lines
packages/vant/src/date-picker/DatePicker.tsx	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12762      +/-   ##
==========================================
+ Coverage   89.76%   89.80%   +0.03%     
==========================================
  Files         257      257              
  Lines        6850     6856       +6     
  Branches     1663     1663              
==========================================
+ Hits         6149     6157       +8     
+ Misses        375      373       -2     
  Partials      326      326              

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