feat: add support for Einsum (Einstein summation)

[pixelspark]

Work in progress... just the parser for now. Some useful resources:

• https://onnx.ai/onnx/operators/onnx__Einsum.html
• https://github.com/termoshtt/einsum-derive/blob/main/einsum-codegen/src/subscripts.rs#L1
• https://stackoverflow.com/questions/26089893/understanding-numpys-einsum