Add IsFirst/IsLast methods to match SS5 conventions (closes silverstr…

…ipe#1274)

src/Models/BaseElement.php

@@ -1231,22 +1231,26 @@ public function getPageTitle()
         return null;
     }
 
-    /**
-     * @return boolean
-     */
-    public function First()
+    public function IsFirst(): bool
     {
         return ($this->Parent()->Elements()->first()->ID === $this->ID);
     }
 
-    /**
-     * @return boolean
-     */
-    public function Last()
+    public function First()
+    {
+        return $this->IsFirst();
+    }
+
+    public function IsLast(): bool
     {
         return ($this->Parent()->Elements()->last()->ID === $this->ID);
     }
 
+    public function Last()
+    {
+        return $this->IsLast();
+    }
+
     /**
      * @return int
      */