feat(avatar): add Avatar component

[North-al]

新增两个组件：

• Avatar：用于展示头像，可为图片、图标或文本，支持自定义大小、形状等；
• AvatarGroup：头像组组件，可用于成员头像组合展示。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.13514% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.68%. Comparing base (ec5b45b) to head (2168c39).
⚠️ Report is 133 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/vant/src/avatar-group/AvatarGroup.tsx	80.00%	3 Missing and 5 partials ⚠️
packages/vant/src/avatar/Avatar.tsx	90.62%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13586      +/-   ##
==========================================
+ Coverage   89.60%   89.68%   +0.08%     
==========================================
  Files         257      261       +4     
  Lines        7013     7117     +104     
  Branches     1736     1769      +33     
==========================================
+ Hits         6284     6383      +99     
- Misses        384      388       +4     
- Partials      345      346       +1     

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