Add support for default ComfyUI "models" path

[flecavalier]

First: thank you for your work on this extension! It's pretty great.

Would you consider moving the default location for model downloads to the standard ComfyUI/models directory, instead of ComfyUI/custom_nodes/SD-Latent-Upscaler/models ?

This would make the model paths more predictable, which would be useful for backup and provisioning purposes (its easier to copy or move the whole ComfyUI/models directory instead of multiple scattered directories across models and custom_nodes)

Having an environment variable to customize could also work (ex: LATENT_UPSCALER_MODELS_PATH). The default value could be ComfyUI/models/latent_upscale for example.

Thank you!