Add missed UndefindValue::Value() method to stubs [skip ci]

Author: pinepain

Diff for: stubs/src/PrimitiveValue.php

@@ -21,5 +21,5 @@
  */
 abstract class PrimitiveValue extends Value
 {
-    abstract function Value();
+    abstract public function Value();
 }

Diff for: stubs/src/UndefinedValue.php

@@ -23,4 +23,11 @@ class UndefinedValue extends PrimitiveValue
     public function __construct(Isolate $isolate)
     {
     }
+
+    /**
+     * @return null
+     */
+    public function Value()
+    {
+    }
 }