Please include .onnx in folder_paths.py

### Custom Node Testing

- [ ] I have tried disabling custom nodes and the issue persists (see [how to disable custom nodes](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled) if you need help)

### Expected Behavior

WanAnimatePreprocess custom node can find .onnx models in the folder

### Actual Behavior

Cannot find .onnx

### Steps to Reproduce

install wananimatepreprocess customnode and try to find .onnx

### Debug Logs

```powershell
As shown in this image
https://private-user-images.githubusercontent.com/205815892/597188112-d7f5e812-f9b4-4cb7-a218-817f5bc9ae81.png?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3ODI3NTAyMjAsIm5iZiI6MTc4Mjc0OTkyMCwicGF0aCI6Ii8yMDU4MTU4OTIvNTk3MTg4MTEyLWQ3ZjVlODEyLWY5YjQtNGNiNy1hMjE4LTgxN2Y1YmM5YWU4MS5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjYwNjI5JTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI2MDYyOVQxNjE4NDBaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT1jMTZlNjcxZDZiMzBjZTc5YjU5YmVlMDZmMjk1OWUwMjIzNjE1NGFkMTkwOWQyNmEzZDEyZDg4OWIwMDdkOWVhJlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZyZXNwb25zZS1jb250ZW50LXR5cGU9aW1hZ2UlMkZwbmcifQ.hJThCkF8y8kGKBUkcvVi42ApeIgYYFzrg8Ej-NfPGOQ
```

### Other

Solution is to just add .onnx as in here
https://github.com/kijai/ComfyUI-WanAnimatePreprocess/issues/33

Open folder_paths.py and trace how model extensions are recognized for custom nodes. Compare the expected .onnx lookup with the WanAnimatePreprocess issue linked in the report, then verify that WanAnimatePreprocess can find .onnx models in its folder.

Written by the indexing model from the issue text.