LibWeb: Parse the border-image-slice property

Author: https://github.com/tcl3
Commit: 245905b833
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5253
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/Forward.h

@@ -177,6 +177,7 @@ class AngleStyleValue;
 class BackgroundRepeatStyleValue;
 class BackgroundSizeStyleValue;
 class BasicShapeStyleValue;
+class BorderImageSliceStyleValue;
 class BorderRadiusStyleValue;
 class CalculatedStyleValue;
 class Clip;