Introduces a Pixi environment for development. Pulls all the dependencies from conda-forge.
Simply clone and run pixi shell -e dev and one can build and compile immediately.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.