[DataGrid] Keyboard navigation with variable row height

[cherniavskii]

Fix keyboard navigation, e.g. Page Up, to use the scroll position and the position of each row

So there's two issues here when it comes to keyboard navigation:

1. PageUp is jumping directly to the column header while not on the first row yet

This was reported here #3218 (comment)

It's reproducible in 5.3.0, so even before variable row height was introduced.

Screen.Recording.2022-01-27.at.19.26.02.mov

Codesandbox: https://codesandbox.io/s/flexlayoutgrid-material-demo-forked-08wux

2. Next rows not loading when navigating with arrow key down when rows have different row height

Screen.Recording.2022-01-27.at.19.17.34.mov

This issue is reproducible in both DataGrid and DataGridPro.
Furthermore, same issue is already present in columns keyboard navigation.
For columns, it's already reported in #3436
The solution seems to be rowThreshold={0}.
Looks like we won't be fixing it before v6. In v6 rowThreshold will be removed.