Add ESM-first dual module support

This change adds full ES Module support while maintaining backward
compatibility with CommonJS.

Key Changes:
- Package is now "type": "module" with ESM as primary format
- Dual compilation: ESM (dist/esm/) and CJS (dist/cjs/)
- Package.json exports map for conditional resolution
- Updated all imports to include .js extensions for ESM compatibility

Module Support:
- ESM: import gocardless from 'gocardless-nodejs'
- CJS: const gocardless = require('gocardless-nodejs')
- Subpath exports: import { parse } from 'gocardless-nodejs/webhooks'
- TypeScript types for both formats
- Tree-shaking support for modern bundlers

Build Output:
- dist/esm/ - ES Modules (primary)
- dist/cjs/ - CommonJS (compatibility layer)
- dist/types/ - Shared TypeScript definitions

Fixes #124

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jamiecobbett

README.md

@@ -20,18 +20,31 @@ To initialise the client, you must provide:
 - The environment that this token is for (see [here](https://github.com/gocardless/gocardless-nodejs/blob/master/src/constants.ts) for a list of available environments).
 - Any additional options (see [here](#available-client-options) for a list of supported options).
 
-<!-- prettier-ignore -->
 ```js
-const gocardless = require('gocardless-nodejs/client');
-const constants = require('gocardless-nodejs/constants');
-
+import gocardless from 'gocardless-nodejs';
+import { Environments } from 'gocardless-nodejs/constants';
 
-// Initialise the client.
+// Initialize the client
 const client = gocardless(
   process.env.GC_ACCESS_TOKEN,
-  constants.Environments.Sandbox,
-  { raiseOnIdempotencyConflict: true },
+  Environments.Sandbox,
+  { raiseOnIdempotencyConflict: true }
+);
+```
+
+TypeScript:
+
+```typescript
+import gocardless, { GoCardlessClient } from 'gocardless-nodejs';
+import { Environments } from 'gocardless-nodejs/constants';
+import type { Payment } from 'gocardless-nodejs/types';
+
+const client: GoCardlessClient = gocardless(
+  process.env.GC_ACCESS_TOKEN,
+  Environments.Sandbox
 );
+
+const payment: Payment = await client.payments.find('PM123');
 ```
 
 ### The Basics
@@ -42,7 +55,7 @@ For a full list of available resources, visit the [GoCardless API reference](htt
 
 <!-- prettier-ignore -->
 ```js
-const uuidv4 = require('uuid/v4');
+import { v4 as uuidv4 } from 'uuid';
 
 // Create a new payment.
 const payment = await client.payments.create(
@@ -86,3 +99,13 @@ for await (const payment of client.payments.all()) {
 ### Available client options
 
 - `raiseOnIdempotencyConflict`: set to `true` to raise exceptions on [idempotency](https://developer.gocardless.com/api-reference/#making-requests-idempotency-keys) conflicts. Defaults to `false`.
+
+
+### CommonJS backwards compatibility
+
+We provide a CommonJS implementation for backwards compatibility. For CommonJS, change the imports to:
+
+```js
+const gocardless = require('gocardless-nodejs');
+const { Environments } = require('gocardless-nodejs/constants');
+```

dist-cjs-package.json

@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}

dist-esm-package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}