Deprecate VectorTemplate

In 1.36 we will be removing this class and adopting SkinMustache.
Signal to developers that this is considered an internal class
as notice of that intention.

Change-Id: Ib62a5352a73eb6189ba85f4034da1b0857fd31aa

includes/VectorTemplate.php

@@ -27,6 +27,8 @@ use Vector\Constants;
 /**
  * QuickTemplate subclass for Vector
  * @ingroup Skins
+ * @deprecated Since 1.35, duplicate class locally if its functionality is needed.
+ * Extensions or skins should extend it under no circumstances.
  */
 class VectorTemplate extends BaseTemplate {
 	/** @var array of alternate message keys for menu labels */