Introduce #[css(function)] for #[derive(ToCss)]

Any variant with this attribute gets serialised as a CSS function,
using the variant name as the function name.

components/style/properties/shorthand/list.mako.rs

@@ -62,7 +62,7 @@
             list_style_type::SpecifiedValue::none
             % else:
             use values::generics::CounterStyleOrNone;
-            list_style_type::SpecifiedValue::CounterStyle(CounterStyleOrNone::None_)
+            list_style_type::SpecifiedValue::CounterStyle(CounterStyleOrNone::None)
             % endif
         }