fix(swiper): keep index when window resizing & code optimizating

[xxyan0205]

背景描述

1. 当窗口尺寸变更时，Swiper会被重新初始化导致index重置。在系统键盘弹起导致的窗口尺寸的场景下[bug-report] swiper组件在屏幕resize的时候会发生重绘，返回第一页 #596，重新初始化会降低使用体验。因此在窗口尺寸时需缓存当前index，保证Swiper停留在当前索引项目上。
2. 其他的代码优化

主要改动

1. window resize的handler中缓存当前index，Swiper初始化以index为准来计算相关尺寸。
2. 原来代码中开始轮播有的调用play，有的调用$_startPlay，将其统一至调用play。
3. play的第一个参数可变更Swiper轮播停留时长，但直接修改prop中的autoplay，会有警告。增加私有变量duration取代autoplay。

需要注意

2，3两点的变动，会给轮播停留时长造成一定影响，需认真review相关代码！！

[codecov-io]

Codecov Report

Merging #597 into dev will increase coverage by 0.26%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev     #597      +/-   ##
==========================================
+ Coverage   81.06%   81.32%   +0.26%     
==========================================
  Files          98       98              
  Lines        2830     2838       +8     
  Branches      552      555       +3     
==========================================
+ Hits         2294     2308      +14     
+ Misses        379      374       -5     
+ Partials      157      156       -1

Impacted Files	Coverage Δ
components/swiper/index.vue	67.27% <66.66%> (+2.7%)	⬆️

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