Add _joinData prop for HABTM.

[dereuromark]

Following #349 cleanup

This resolves #112

Can anyone please verify this?

Index: src/Model/Entity/Article.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/src/Model/Entity/Article.php b/src/Model/Entity/Article.php
--- a/src/Model/Entity/Article.php	(revision 3bb46010a6347b95739c6d6dac4db125a40613fb)
+++ b/src/Model/Entity/Article.php	(date 1711459930687)
@@ -112,6 +112,7 @@
  * @property int|null $count
+ * @property \Cake\ORM\Entity $_joinData
  */
 class Article extends Entity
 {