add pixi env

[matthdsm]

PR checklist

Closes #XXX

• This comment contains a description of changes (with reason).
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[matthdsm]

Feel free to hit merge if there are no remaining issues

[mashehu]

maybe add docs on how to use this in the readme or contributing.md

[mashehu]

should we switch the devcontainer to also use this then, to keep them in sync?

[matthdsm]

Added an attempt, not sure if it'll work/ is correct

[mashehu]

cc @JulianFlesch can you have a look at the devcontainer please?