Fix first/last item margin on scroll (#8880)

![output](https://github.com/zed-industries/zed/assets/5518/e39a3600-99c4-4d3c-baee-efd53a474f38)

Before:

https://github.com/zed-industries/zed/assets/5518/f7a4563a-504a-4a41-bfd4-21e9439cd02b

After:

https://github.com/zed-industries/zed/assets/5518/0ba41527-46fd-404f-8207-1b8c5cf37434


Release Notes:

- Fixed first and last item margin when scroll view has padding