Add outlines for .css files

⚠️I also added `workspace = { workspace = true, features =
["test-support"] }` to dev deps of 'languages' crate to make unit tests
work.

![Css
outlines](https://github.com/zed-industries/zed/assets/4057095/0cade407-79e7-4d0f-9b80-3502509e373d)