This makes sure that the `vertical_scroll_margin` doesn't leave the
cursor out of screen or somewhere it shouldn't go when it's higher than
the visible lines on screen.
So we cap it to `visible_line_count / 2`, similar to nvim:
5aa1a9532c/src/nvim/window.c (L6560)Fixes#15101
Release Notes:
- Fixed `ctrl-u`/`ctrl-d` in Vim mode not working correctly when
`vertical_scroll_margin` is set to a really high value.
Co-authored-by: Bennet <bennet@zed.dev>
c1872e9cb0