Add method isListKey

Author: ahjdev

src/Types/AbstractList.php

@@ -61,6 +61,14 @@ public function getValueType(): Type
         return $this->valueType;
     }
 
+    /**
+     * Returns key is a list
+     */
+    public function isListKey(): bool
+    {
+        return $this->keyType !== null;
+    }
+
     /**
      * Returns a rendered output of the Type as it would be used in a DocBlock.
      */