Add CSS at-rules to identifier regex

CSS also has things that start with `@`, such as `@import`, `@media`,
and `@keyframes`. It might make sense to add a `@` to the beginning of
the CSS identifier regex.

MDN lists the CSS at-rules as: CSS `@charset`, `@document`,
`@font-face`, `@import`, `@keyframes`, `@media`, `@page`, `@supports`.