[RFC] Discount getters

packages/core/rfcs/discount-getters.md

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+# Discount / coupons getters
+
+## Motivation
+
+There is no possibility to read discounts from the cart. We are not able to display discounted total price or even an information about applied discount code. Suggestion bellow:
+
+```ts
+export interface CartGetters<CART, CART_ITEM, DISCOUNT> {
+  // ...
+  // new ones:
+
+  getDiscount: () => DISCOUNT; // don't think if it's needed
+  getDiscountId: () => string;
+  getDiscountedTotals: () => AgnosticTotals;
+  getDiscountName: () => string;
+  getDiscountValue: () => number;
+  getDiscountType: () => string; // fixed or percentage
+
+  [getterName: string]: (element: any, options?: any) => unknown;
+}
+```
+
+Also I'm thinking about separated getters, which has even more sense as you can have multiple discounts applied to the cart.
+
+```ts
+export interface DiscountGetters {
+  getId: () => string;
+  getName: () => string;
+  getValue: () => number;
+  getType: () => string; // fixed or percentage
+}
+
+export interface CartGetters<CART, CART_ITEM, DISCOUNT> {
+  // ...
+  // new ones:
+
+  getDiscounts: () => DISCOUNT[]; // now you can use discountGetters on each
+  getDiscountedTotals: () => AgnosticTotals; // still useful
+
+  [getterName: string]: (element: any, options?: any) => unknown;
+}
+```
+
+And also we don't need a coupon property anomore:
+
+```ts
+export interface UseCart {
+  // ...
+  coupon: ComputedProperty<COUPON | null>; // to be removed
+  // ...
+}
+```
+
+## Migration process
+
+Prety straightforward, just implement a new getters.