LibJS: Add Reference class to represent a base.property reference

Expression nodes can now be asked to produce a Reference. We then use
this to implement the "delete" operator without downcasting the child
node to a MemberExpression manually.

Libraries/LibJS/Forward.h

@@ -66,6 +66,7 @@ class Interpreter;
 class LexicalEnvironment;
 class MarkedValueList;
 class PrimitiveString;
+class Reference;
 class ScopeNode;
 class Shape;
 class Statement;