Wrappers for additional tblis functions

[chillenb]

1. Pip 25 will refuse to do editable installs without a pyproject.toml file. I added a minimal one according to the recommendations in the linked issue.
2. Added wrappers for tblis_tensor_add and tblis_tensor_dot
3. Renamed the C function as_einsum to tensor_mult. This does not change the public API, but I can drop this point and keep the original name if some code depends on it.
4. Fixed RPATH settings when external tblis is linked.