LibJS: Make Value() default-construct the undefined value

The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().

The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.

Libraries/LibJS/Runtime/FunctionEnvironment.h

@@ -34,7 +34,7 @@ public:
     Value new_target() const { return m_new_target; }
     void set_new_target(Value new_target)
     {
-        VERIFY(!new_target.is_empty());
+        VERIFY(!new_target.is_special_empty_value());
         m_new_target = new_target;
     }