StripeKit - Apply Subscription Promotion Code

`kit.subscriptions.applyPromotionCode()`

Applies a coupon's promotion code to an already-active subscription, discounting future invoices.

Field	Type	Description
`subscriptionId`	`string`	The subscription to discount.
`stripePromotionCodeId`	`string`	A Stripe promotion code ID (starts with `promo_`), not the plain-text code a customer types.

Same shape as retrieve()'s response.

Exception	When
`StripeOperationError`	The subscription or promotion code ID does not exist, or the promotion code is no longer active.

If a storage adapter is configured, the record is persisted via saveSubscription() after applying.

This overwrites any promotion code currently applied to the subscription; Stripe subscriptions can only have one active promotion code at a time.