`pixi --strict` mode about pixi

[Hofer-Julian]

I think we should be conservative when adding new options. However, maybe we could just error here?

[nichmor]

good point - but I think it could be a potential breaking change for users that have unused features, and after running pixi self-update or having setup-pixi: latest CI will receive some red pipelines

[olivier-lacroix]

Optional dependencies in pyproject.toml are interpreted as features. There are likely situations where not having an environment for each feature is desired. So defaulting to erroring could be an issue I think?

I feel like the current warning is a good middle ground for most use cases. A strict mode / check command may help in some other situations