Refactor evolve and evolveWithMetadata to avoid code duplication and a potential bug for amounts

[shraddha38]

Summary:

The evolve and evolveWithMetadata functions in ShoppingCart have some code duplication and lack safety for new event types.

Suggested Improvements

1. Removing duplication - Reduce duplication in event handling logic and ensure safety by adding a default case in switch statements

Same logic is used in evolve and evolveMetatadata fn, so we can extract it as following -

const updateCart = (state: ShoppingCart, productItem: PricedProductItem): ShoppingCart => ({
  productItems: [...state.productItems, productItem],
  totalAmount: state.totalAmount + productItem.price * productItem.quantity,
});

2. Bug- Prevent discounts from causing negative amounts

Current

 case 'DiscountApplied':
      return {
        ...state,
        totalAmount: state.totalAmount * (1 - data.percent / 100),
      };

Suggestion

 case 'DiscountApplied':
      return {
        ...state,
        totalAmount: Math.max(state.totalAmount * (1 - data.percent / 100), 0),
      };

Next Steps

1. Need review from maintainers on this issue.
2. After that I can create a Pr for the changes.