style: Add some attributes for SpecifiedValueInfo to help deriving more from types.

Bug: 1434130
Reviewed-by: emilio
MozReview-Commit-ID: IyohSTbUO31

components/style/values/generics/effects.rs

@@ -19,6 +19,7 @@ pub struct BoxShadow<Color, SizeLength, BlurShapeLength, ShapeLength> {
     pub spread: ShapeLength,
     /// Whether this is an inset box shadow.
     #[animation(constant)]
+    #[value_info(represents_keyword)]
     pub inset: bool,
 }