LibWeb: Add place-self css property support

Author: https://github.com/kalenikaliaksandr
Commit: da2cd73bcf
Pull-request: https://github.com/SerenityOS/serenity/pull/20359
Reviewed-by: https://github.com/AtkinsSJ

Userland/Libraries/LibWeb/Forward.h

@@ -149,6 +149,7 @@ class PercentageOrCalculated;
 class PercentageStyleValue;
 class PlaceContentStyleValue;
 class PlaceItemsStyleValue;
+class PlaceSelfStyleValue;
 class PositionStyleValue;
 class PropertyOwningCSSStyleDeclaration;
 class RadialGradientStyleValue;