## Context

This is an implementation of this feature:
https://github.com/zed-industries/zed/discussions/49821

Although, I'd argue it's a bug fix, not a feature.

Either way : This copies the window scroll logic from the `vim` mode
versions with out all the extra logic for visual mode.

Could refactor the common logic out of the vim code and make it common.
But that seems like a bigger PR. Happy to take a stab at it if that's
what you prefer.
Could also add new commands for the new behavior if you prefer. I didn't
do that, because it seems like more clutter in the commands, and my
belief that the existing behavior is a bug. But happy to do that if you
prefer.

## How to Review

creates new function `scroll_screen_with_cursor_margin` in `scroll.rs`
wires up editor::LineUp/LineDown to new function in `elements.rs`
adds a test in  `editor_tests.rs`

## Self-Review Checklist

<!-- Check before requesting review: -->
- [X] I've reviewed my own diff for quality, security, and reliability
- [X] Unsafe blocks (if any) have justifying comments
- [X] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [X] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Release Notes:

- editor::LineUp/LineDown commands now honor vertical_scroll_margin
(same as vim::LineUp/LineDown)

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Co-authored-by: Kirill Bulatov <mail4score@gmail.com>