Another PowerShell egg-timer (using events)

Recording a learning exercise for how PowerShell handles events, and how to communicate into event handlers

Note that every registered event must be unregistered (otherwise running the script leaves droppings in your PowerShell session in the form of those claims on events). The events that are handled -- the timer 250ms wake-ups -- are consumed by being handled; but the one that is merely waited on remains latched after triggering the wait to release and must be explicitly cleared.

Additional AV frills would be the same as any of the polling loop timer examples out there.