[bugfix] Step: incorrect active step when insert step asynchronously

Merge branch 'dev'

[release] 1.6.15

[bugfix] Step: step 销毁之前从父 steps 中移除 (youzan#3140)

[Doc] Stepper: optimize async demo (youzan#3098)

[bugfix] Picker: render error when set data async (youzan#3064)

[new feature] Sku: set selectedNum when inited (youzan#3045)

[new feature] Icon: add tag prop (youzan#2986)

[new feature] Collapse: add size prop (youzan#2946)

[improvement] Picker: shared props (youzan#2888)