feat(Selector):add `gap` attribute

[MuxinFeng]

related issue:https://github.com/ant-design/ant-design-mobile/issues/5549

[codecov[bot]]

Codecov Report

Merging #5554 (e42d358) into master (25c7aca) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5554      +/-   ##
==========================================
- Coverage   89.82%   89.82%   -0.01%     
==========================================
  Files         294      294              
  Lines        6251     6249       -2     
  Branches     1529     1528       -1     
==========================================
- Hits         5615     5613       -2     
  Misses        589      589              
  Partials       47       47              

Impacted Files	Coverage Δ
src/components/selector/selector.tsx	100.00% <100.00%> (ø)

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[github-actions[bot]]

PR preview has been successfully built and deployed to https://antd-mobile-preview-pr-5554.surge.sh

[MuxinFeng]

改好了，麻烦大佬再cc呢

[awmleer]

要不把这些 CSS 变量也暴露出来吧……？这样就和 Grid 完全一致了

--gap
--gap-horizontal
--gap-vertical

[MuxinFeng]

好滴

[MuxinFeng]

emmm，大佬不好意思，这些样式属性我目前加不进去。我参考了其他组件的写法（如Space、Stepper），可还是一头雾水，如果这部分不紧急的话，是否可以后面再加进去呢~

[miracles1919]

CSS 变量我加上去了

[MuxinFeng]

学习了~