input: Write a python function to add a node to the end of the linked list.
  assert add_node_to_end(LinkedList(), 1).head.data == 1
  assert add_node_to_end(LinkedList(), 1).head.next == None
  ll = LinkedList()
  ll = add_node_to_end(ll, 1)
  ll = add_node_to_end(ll, 2)
  assert ll.head.next.data == 2
output: class Node:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  class LinkedList:
      def __init__(self):
          self.head = None
  
  def add_node_to_end(ll, data):
      new_node = Node(data)
      if ll.head is None:
          ll.head = new_node
      else:
          current = ll.head
          while current.next is not None:
              current = current.next
          current.next = new_node
      return ll