support pixi

[keewis]

With pixi, we can also define minimum version environments, but they're not in the same format. In theory we could just parse pyproject.toml then find the environment and merge the extras.

However, pixi has a convenient command for this:

pixi list --frozen -x -e <env> --json

which will give us all the info in json format. This does require pixi to be installed, but that's something we can just conditionally install.

I'm not sure how to best encode this, though: we could rename the options to conda-environment-files and pixi-environments (and keep the old environment-files for backwards compat), but that would require us to somehow make all three mutually exclusive, which I'm not sure is possible.