AK: Rename Vector::append(Vector) => Vector::extend(Vector)

Let's make it a bit more clear when we're appending the elements from
one vector to the end of another vector.

Userland/Libraries/LibJS/Runtime/MarkedValueList.h

@@ -28,7 +28,7 @@ public:
     MarkedValueList copy() const
     {
         MarkedValueList copy { m_heap };
-        copy.append(*this);
+        copy.extend(*this);
         return copy;
     }