input: Write a python function to convert a binary search tree to a list in ascending order.
  assert bst_to_list(root) == [5, 13, 24, 42, 76, 90]
  assert bst_to_list(insert(None, 10)) == [10]
  assert bst_to_list(None) == []
output: def bst_to_list(root):
      def inorder_traversal(node, result):
          if node:
              inorder_traversal(node.left, result)
              result.append(node.val)
              inorder_traversal(node.right, result)
      
      result = []
      inorder_traversal(root, result)
      return result