Add/update the quantized ONNX model files and README.md for Transformers.js v3

## Applied Quantizations

### ✅ Based on `decoder_model_merged.onnx` *with* slimming

↳ ✅ `fp16`: `decoder_model_merged_fp16.onnx` (replaced because it was invalid)
↳ ✅ `int8`: `decoder_model_merged_int8.onnx` (added)
↳ ✅ `uint8`: `decoder_model_merged_uint8.onnx` (added)
↳ ✅ `q4`: `decoder_model_merged_q4.onnx` (added)
↳ ✅ `q4f16`: `decoder_model_merged_q4f16.onnx` (added)
↳ ✅ `bnb4`: `decoder_model_merged_bnb4.onnx` (added)

onnx/decoder_model_merged_bnb4.onnx

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onnx/decoder_model_merged_fp16.onnx

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onnx/decoder_model_merged_int8.onnx

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onnx/decoder_model_merged_q4.onnx

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onnx/decoder_model_merged_q4f16.onnx

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onnx/decoder_model_merged_uint8.onnx

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