Add pixi/config.toml with pinning strategy

[irm-codebase]

Describe the bug

The file in the title is missing because .gitignore in that folder lists it as config.yaml.

config.toml should have:

pinning-strategy = "latest-up"

To Reproduce
Look at the files?

Expected behavior
Adding new dependencies with pixi add should not put a maximum version unless absolutely necessary to avoid breaking dependencies for users.