Add time of day-based greetings

[XVII]

It would be useful to have a helper function for building Greetings, i.e. "Good morning", "Good afternoon", "Good evening" based on the current time.

Perhaps an extension to DateTime and TimeOnly along the lines of .ToPeriodOfDayNotation() might be the best way to implement this. It would return "morning" or "afternoon" or "evening" which could then be adapted into a greeting.

Or, could be .ToGreetingNotation() perhaps?