theme: Warn when deprecated `scrollbar_thumb.background` style is used (#13081)

This PR adds a warning when the deprecated `scrollbar_thumb.background`
style property is present in a theme.

This property has been succeeded by `scrollbar.thumb.background`.

The primary reason for this is to get it into the `zed-extension` CLI so
that we can use it to detect which themes need to be updated.

Release Notes:

- N/A