Short-hand expression for Constant Delay

[stephanbuys]

As per author: "Constant Delay; an extension I've seen that is neat is @every 1h to define a delayed task instead of a scheduled task"

[mehcode]

This is sort of done. The short-hand syntax is not there but the constant delay is. Going to change this issue to focus on the syntax.

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my_agenda.add(|| { }).schedule(Duration::seconds(10));