Add ONNX export capability for CREPE models

This commit adds support for exporting CREPE models to ONNX format:

1. Add export_model_to_onnx() function to core.py that converts Keras models to ONNX
2. Create command-line script (onnx_export.py) to easily export models with different capacities
3. Update .gitignore to exclude generated ONNX model files

The ONNX export enables using CREPE models in non-TensorFlow environments and optimizing with tools like ONNX Runtime for deployment across various platforms.

Usage examples:

python -m crepe.onnx_export tiny  # Creates model-tiny.onnx
python -m crepe.onnx_export full -o custom_name.onnx

example usage here: https://github.com/joeloftusdev/crepe.cpp