input: Write a function to append an element to a linked list.
  assert isinstance(linked_list.head, Node) == True
  linked_list.append(10)
  assert linked_list.head.data == 10
  linked_list.append(20)
  assert linked_list.head.next.data == 20
  linked_list.append(30)
  assert linked_list.head.next.next.data == 30
output: class Node:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  class LinkedList:
      def __init__(self):
          self.head = None
  
      def append(self, data):
          new_node = Node(data)
          if not self.head:
              self.head = new_node
              return
          last = self.head
          while last.next:
              last = last.next
          last.next = new_node