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/plugin/utils.ts

@@ -1,5 +1,6 @@
-import type { CollectionConfig, Field } from 'payload'
+import type { CollectionConfig, CollectionSlug, Field } from 'payload'
 
 export type FieldsOverride = (args: { defaultFields: Field[] }) => Field[]
 
-export const extractSlug = (arg: string | Partial<CollectionConfig>) => typeof arg === 'string' ? arg : arg.slug!
+export const extractSlug =
+  (arg: string | Partial<CollectionConfig>) => (typeof arg === 'string' ? arg : arg.slug!) as CollectionSlug