LibWeb: Implement new StyleValueList

StyleValueList is a list of StyleValues of the same type, for use in
properties like `margin` which accept a variable number of arguments.

I had originally hoped to simply swap the old ValueListStyleValue from
being a list of ComponentValues to one of StyleValues, but I can see now
that I will need to have both for a little while, so renamed the old
is_value_list() to is_component_value_list() temporarily.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -173,7 +173,7 @@ void ImageStyleValue::resource_did_load()
 }
 
 ValueListStyleValue::ValueListStyleValue(Vector<StyleComponentValueRule>&& values)
-    : StyleValue(Type::ValueList)
+    : StyleValue(Type::ComponentValueList)
     , m_values(move(values))
 {
 }