LibJS: Implement the "instanceof" operator

This operator walks the prototype chain of the RHS value and looks for
a "prototype" property with the same value as the prototype of the LHS.

This is pretty cool. :^)

Libraries/LibJS/AST.h

@@ -302,6 +302,7 @@ enum class BinaryOp {
     BitwiseXor,
     LeftShift,
     RightShift,
+    InstanceOf,
 };
 
 class BinaryExpression : public Expression {