visualization: style `async` nodes

A `Driver` can successfully build a dataflow with async nodes, but will error out on `Driver.execute()`. This `Driver` is still useful because it allows to have `dr.display_all_functions()` to quickly view the DAG.

However, it would be useful to style `async` nodes differently to remind that the `AsyncDriver` is required to execute the code.

(requested by a user)

Start by tracing the visualization path used by dr.display_all_functions() and how it represents nodes built by Driver, then compare the distinction from AsyncDriver and Driver.execute(). Done means async nodes are visibly styled differently in the displayed DAG while the existing view remains usable.

Written by the indexing model from the issue text.