css.lang: Combine at-rule keyword and call contexts
css.lang: Combine at-rule keyword and call contexts

This brings the highlighting of each at-rule and its calling syntax
together, removing matching duplication.

This also sets keyword-char-class, primarily so that hyphens are taken
into account when testing for word boundaries (hyphens are valid within
CSS identifiers).

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview