feat: Add Mollie payment provider support

- Introduce `mollieProvider` for handling Mollie payments
- Add configurable payment hooks for initialization and processing
- Implement `initPayment` logic to create Mollie payments and update metadata
- Include types for Mollie integration in payments and refunds
- Update `package.json` to include `@mollie/api-client` dependency
- Refactor existing payment-related types into modular files for better maintainability

src/index.ts

@@ -1,2 +1,4 @@
 
-
+export { billingPlugin } from './plugin'
+export type { BillingPluginConfig, CustomerInfoExtractor } from './plugin/config'
+export type { Invoice, Payment, Refund } from './plugin/types'