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/StyleDeclaration.h

@@ -4,7 +4,7 @@
 #include <LibHTML/CSS/StyleValue.h>
 
 struct StyleProperty {
-    String name;
+    CSS::PropertyID property_id;
     NonnullRefPtr<StyleValue> value;
     bool important { false };
 };