Fixed climbing blocks such as ladders and vines

Seems we can now climb ANY block if the climbing flag is true, and nothing if false. This commit adds local block checks to see if a climbable block exists at the entity's feet and if so, sets the flag.

src/pocketmine/block/Block.php

@@ -472,6 +472,14 @@ class Block extends Position implements BlockIds, Metadatable{
 		return false;
 	}
 
+	/**
+	 * Returns whether entities can climb up this block.
+	 * @return bool
+	 */
+	public function canClimb() : bool{
+		return false;
+	}
+
 	/**
 	 * @return string
 	 */