Replace deprecated scrollbar_thumb.background with scrollbar.thumb.background

[injust]

From Zed logs:

[WARN] Theme "Catppuccin Latte" is using a deprecated style property: scrollbar_thumb.background. Use `scrollbar.thumb.background` instead.
[WARN] Theme "Catppuccin Frappé" is using a deprecated style property: scrollbar_thumb.background. Use `scrollbar.thumb.background` instead.
[WARN] Theme "Catppuccin Macchiato" is using a deprecated style property: scrollbar_thumb.background. Use `scrollbar.thumb.background` instead.
[WARN] Theme "Catppuccin Mocha" is using a deprecated style property: scrollbar_thumb.background. Use `scrollbar.thumb.background` instead.

[injust]

Tested by installing dev extension. No noticeable changes but the warning disappeared from the log.

[tecandrew]

nice! lgtm. thanks for doing this. haven't been too aware of their schema changes

i'll merge this and your other one and bump the version this weekend. all other accent flavors need to be regenerated for the GH release