fix(cdk:scroll): virtual scroll start offset should add size of prepended items

[sallerli1]

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• The commit message follows our guidelines
• Tests for the changes have been added/updated or not needed
• Docs and demo have been added/updated or not needed

What is the current behavior?

当虚拟滚动传入了 colModifier 和 rowModifier 时，滚动的offset值应当加上被添加到行首或者列首的元素尺寸，保证滚动的内容区的offset正确，否则会造成滚动位置跳跃的情况

What is the new behavior?

修复以上问题

Other information

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.70%. Comparing base (9c81358) to head (7913777).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1950   +/-   ##
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  Coverage   90.70%   90.70%           
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  Files         360      360           
  Lines       35595    35595           
  Branches     3863     3863           
=======================================
  Hits        32286    32286           
  Misses       3309     3309           

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