LibHTML: Use an enum for CSS property ID's

Instead of using string everywhere, have the CSS parser produce enum
values, since they are a lot nicer to work with.

In the future we should generate most of this code based on a list of
supported CSS properties.

Libraries/LibHTML/CSS/StyleValue.h

@@ -6,6 +6,7 @@
 #include <AK/StringView.h>
 #include <LibDraw/Color.h>
 #include <LibHTML/CSS/Length.h>
+#include <LibHTML/CSS/PropertyID.h>
 
 class Document;