feat(export): add buffer download methods to ExportBuilder

- Added download() method to get statements as Buffer without saving to disk
- Added downloadAsArrayBuffer() method for web API compatibility
- Enhanced documentation for getAccountStatement() method
- Updated README with comprehensive examples
- No breaking changes, backward compatible

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@apiclient.xyz/bunq",
-  "version": "4.3.0",
+  "version": "4.4.0",
   "private": false,
   "description": "A full-featured TypeScript/JavaScript client for the bunq API",
   "type": "module",