Similar to &zip_xy_scaler, add a &zip_scroll_scaler to scale wheel/hwheel movements. There is also both &zip_xy_transform and &zip_scroll_transform, so it seems to make sense to have &zip_scroll_scaler pre-defined.

Also take the opportunity to add some mouse scroll tests, including for &zip_scroll_scaler and &zip_scroll_transform.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.