LibWeb: Implement <center> as -libweb-center

To get the expected behavior for <center>, we needed a special text
alignment mode that centers block-level elements (and not just line
box fragments.)

Libraries/LibWeb/CSS/StyleValue.h

@@ -104,6 +104,7 @@ enum class ValueID {
     Left,
     Right,
     Justify,
+    VendorSpecificCenter,
 };
 
 enum class Position {