refactor: Make the Holiday class implement the Stringable interface

The 'Holiday' class already includes the '__toString' method, so good to
have it implement the Stringable interface.

Signed-off-by: Sacha Telgenhof <[email protected]>

src/Yasumi/Holiday.php

@@ -23,7 +23,7 @@
 /**
  * Class Holiday.
  */
-class Holiday extends \DateTime implements \JsonSerializable
+class Holiday extends \DateTime implements \JsonSerializable, \Stringable
 {
     /**
      * Type definition for Official (i.e. National/Federal) holidays.