editor: Fix autoscroll flickering regression (#24758)

This PR fixes autoscroll flickering issue caused by recent
[#24735](https://github.com/zed-industries/zed/pull/24735) which fixes
soft wrap scroll issues. No release notes, as this was few hours ago.

Adding vertical scrollbar width to viewport width, so that autoscroll
function don't try to that much pixels extra.

Release Notes:

- N/A