runOnExit abstract method

Registers handler as the onExit handler function for this state.

This method supports adding 'open-coded' handlers to the state. Because the handler function was not described calling builder methods, the specific behavior of the method will be opaque to a StateTreeFormatter when StateTreeBuilder.format is called. As a result, the graph description produced by the formatter may not be particularly useful. This method is best avoided if the formatting feature is important to you.