style: Implement <meta name=color-scheme>

There are still tests failing because
https://bugzilla.mozilla.org/show_bug.cgi?id=1736034 hasn't been synced
yet.

Once that lands, they will still fail because we don't change
Canvas/CanvasText based on color-scheme, but that I'm attaching
patches for after this one.

Differential Revision: https://phabricator.services.mozilla.com/D129743

components/style/values/specified/color.rs

@@ -958,6 +958,11 @@ impl ColorScheme {
             bits: ColorSchemeFlags::empty(),
         }
     }
+
+    /// Returns the raw bitfield.
+    pub fn raw_bits(&self) -> u8 {
+        self.bits.bits
+    }
 }
 
 impl Parse for ColorScheme {