Add pixi support and CI

[ManifoldFR]

This PR adds pixi support and CI.

Merge after: #27

[ManifoldFR]

This change is part of the following stack:

• Change all instances of taking argument "const Eigen::VectorXd &" to "const ConstVectorRef&" #27
  • Add pixi support and CI #28 ◀

_{Change managed by git-spice.}