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/collections/invoices.ts

@@ -5,10 +5,10 @@ import {
   CollectionBeforeValidateHook,
   CollectionConfig, Field,
 } from 'payload'
-import type { BillingPluginConfig} from '@/plugin/config';
-import { defaults } from '@/plugin/config'
-import { extractSlug } from '@/plugin/utils'
-import type { Invoice } from '@/plugin/types/invoices'
+import type { BillingPluginConfig} from '../plugin/config.js';
+import { defaults } from '../plugin/config.js'
+import { extractSlug } from '../plugin/utils.js'
+import type { Invoice } from '../plugin/types/invoices.js'
 
 export function createInvoicesCollection(pluginConfig: BillingPluginConfig): CollectionConfig {
   const {customerRelationSlug, customerInfoExtractor} = pluginConfig