style: Add system-ui boilerplate

Alias -apple-system to it, and put it behind a pref for now. This is
pretty boring (read: uncontroversial hopefully) code. The follow-up work
is modifying StaticPresData to look up the fonts using system APIs,
probably. Maybe a bit more work if on macOS they can't be named.

Differential Revision: https://phabricator.services.mozilla.com/D119984

components/style/properties/shorthands/font.mako.rs

@@ -137,7 +137,7 @@
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError))
         }
 
-        let family = FontFamily::parse_specified(input)?;
+        let family = FontFamily::parse(context, input)?;
         Ok(expanded! {
             % for name in "style weight stretch variant_caps".split():
                 font_${name}: unwrap_or_initial!(font_${name}, ${name}),