[improvement] Swipe: add indicator slot

[jerryni]

修改：

• 添加名称为indicator的slot
• 默认值之前的指示器的样式
• 添加了符合vant风的demo

[codecov-io]

Codecov Report

Merging #1923 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1923   +/-   ##
=======================================
  Coverage   81.16%   81.16%           
=======================================
  Files          96       96           
  Lines        1667     1667           
  Branches      335      335           
=======================================
  Hits         1353     1353           
  Misses        260      260           
  Partials       54       54

Impacted Files	Coverage Δ
packages/swipe/index.vue	95.23% <ø> (ø)	⬆️

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