update_adj_list: Update adjacency list.
In autoharp: Semi-Automatic Grading of R and Rmd Scripts

Updates the adjacency list for an R expression parse tree.

update_adj_list(
  update_type = c("new_node", "add_child"),
  node_id,
  node_name,
  child_node,
  env_ni
)

`update_type`	This should be either "new_node" or "add_child". If it is a new node, an empty list component is added. If it is add child, then child_node should be provided too.
`node_id`	An integer.
`node_name`	The name of the new node to be added. This must be provided if the update_type is "new_node".
`child_node`	An integer.
`env_ni`	An environment object, possibly containing an adjacency list that will later be used to construct a TreeHarp object.