feat(exec): add conda-script support

[wolfv]

Finally got a chance to vibe code pixi script support (https://github.com/prefix-dev/pixi/issues/3751).

You can find examples under examples/.

And run them with pixi exec examples/conda-script/hello_python.py.

Would love to hear if this is in line with the proposal.

I thought we had decided to go for something truly PEP 723 and make [dependencies] use PyPI dependencies, and have another conda or pixi dependencies table. Maybe you can update me on that discussion @ruben-arts / @nichmor.

PR was fully executed by Claude.

[baszalmstra]

This is also missing documentation and integration tests.

I also think we should align the syntax with environment.yaml v2.

[ruben-arts]

For the python world I think it's best to also support the pep

[ruben-arts]

• Should we default to conda-forge in this case, like pixi exec normally does?
• Do we need entrypoint or is it just a feature? When I remove it it gives me a "no such file or directory" error.
• Do the files need to be executable?