Decorator

Attach new behavior to an object dynamically, by wrapping it, instead of subclassing every combination (CoffeeWithMilk, CoffeeWithMilkAndSugar, ...).

Implement `MilkDecorator`/`SugarDecorator`, each wrapping a Coffee and adding to its cost()/description().

solve(addons) wraps a SimpleCoffee (cost 2, description 'Coffee') with one decorator per addon, in order, and returns the final { cost, description }.

solve(['milk', 'sugar']) → { cost: 2.75, description: 'Coffee + Milk + Sugar' }