setup.sh: add support for Mamba

[AlexTate]

The setup script can detect and utilize existing Conda installations in favor of performing a Miniconda installation, but it doesn't do the same for Mamba or Micromamba. The future of Mamba as a drop-in replacement for Conda is unclear because starting with Conda v23.10.0, libmamba is now the default solver for Conda so the performance gap for solving environments is much narrower (if not negligible) between the two. However, the Mamba project has gained a large following and may still be advantageous in other ways, so we can support them as long as they maintain their goal as a drop-in replacement for Conda.