Mayors are people too.
An instance of the Mayor type will naturally get nervous whenever its town loses some population due to a Zombie attack.
Create a stored instance property on the Mayor type called anxietyLevel.
It should be of type Int and should start out with a default value of
Increment the anxietyLevel property every time a Mayor instance is notified of a Zombie attack.
Last, as a mayor will not want to outwardly display anxiety, mark this property as
Verify that this property is not accessible in main.swift.