LibHTML: Parse descendant relations in CSS selectors

"div p" now generates a Selector with two components where the second
component is a type=TagName, value="p", relation=Descendant.

We still don't handle matching of these, but at least we parse them.

Libraries/LibHTML/CSS/Selector.h

@@ -18,6 +18,7 @@ public:
         enum class Relation {
             None,
             ImmediateChild,
+            Descendant,
         };
         Relation relation { Relation::None };