fix(Tab): tab(with sticky prop) loss 'fixed' classname after switch tab

[shenjingnan]

fix: #12535

边界场景(需要出现两个吸顶)

调整 <Sticky /> 判断 offset === root top 时的吸顶逻辑：
调整前：逻辑是两者相等时认为可以解除吸顶
调整后：逻辑是保持吸顶，直到 offset < root top(即用户开始往上滚内容👆🏻)

这样做能兼容 <Tabs sticky /> 当用户什么都不做时，只是切换吸顶tab时的情况(切换tab时offset === root top)

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issue 本身使用的也有问题，<VanNavBar fixed title="pageTitle" /> 本身也是固定在顶部的，必定会出现重叠的情况，最好的做法是增加 tab 偏移量，例如 <Tabs sticky :offset-top="46" />

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b5645ee) 89.88% compared to head (19a6d32) 89.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12547      +/-   ##
==========================================
+ Coverage   89.88%   89.91%   +0.03%     
==========================================
  Files         257      257              
  Lines        6819     6820       +1     
  Branches     1653     1653              
==========================================
+ Hits         6129     6132       +3     
+ Misses        367      365       -2     
  Partials      323      323              

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[chenjiahan]

LGTM 😄