[BUG] Esm Configuration Files are not found

**Describe the bug**
Configuration files with the file extension mjs are not found by this library when setting the NODE_APP_INSTANCE environment variable.

**Expected behavior**
Configuration files with the file extension mjs should be found by the library when setting the NODE_APP_INSTANCE environment variable.

 -  node-config version: 4.0.1
 -  node-version: 22.17.1
 
### Other information
