LibHTML: Add adjacent (+) and general (~) sibling combinators

This patch implements two more selector features:

- "div + p" matches the <p> sibling immediately after a <div>.
- "div ~ p" matches all <p> siblings after a <div>.

Libraries/LibHTML/CSS/Selector.h

@@ -19,6 +19,8 @@ public:
             None,
             ImmediateChild,
             Descendant,
+            AdjacentSibling,
+            GeneralSibling,
         };
         Relation relation { Relation::None };