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

[sallerli1]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added/updated or not needed
• [x] 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 Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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