fix: resolve module resolution errors by replacing path aliases with relative imports

- Replace all @/ path aliases with proper relative imports and .js extensions
- Update @mollie/api-client peer dependency to support v4.x (^3.7.0 || ^4.0.0)
- Bump version to 0.1.5
- Ensure ESM compatibility for plugin distribution

Fixes module resolution error: "Cannot find package '@/collections'" when using
the plugin in external PayloadCMS projects.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/providers/types.ts

@@ -1,6 +1,6 @@
-import type { Payment } from '@/plugin/types/payments'
+import type { Payment } from '../plugin/types/payments.js'
 import type { Config, Payload } from 'payload'
-import type { BillingPluginConfig } from '@/plugin/config'
+import type { BillingPluginConfig } from '../plugin/config.js'
 
 export type InitPayment = (payload: Payload, payment: Partial<Payment>) => Promise<Partial<Payment>>