[Currency] Improve CurrencyPlugin to deal better with duplicate currencies

The CurrencyPlugin provides helper method that deal with lists of currencies. However this could lead to the following odd scenario

const currency = new CurrencyPlugin([{ id: 'money' }]);
currency.gainCurrency({ id: 'money', amount: 1 });

const isPaid = currency.payCurrencies([
  { id: 'money', amount: 1 },
  { id: 'money', amount: 1 },
]);

console.log(isPaid); // true
console.log(currency.getBalance('money')); // -1

We could merge these lists together with a new simplifyCurrencyList function before checking, so it becomes

[{ id: 'money', amount: 2 }]