Provide `default` environment next to `base`

[jaimergp]

Prototype installer at Miniforge:

•  Define default extra env configured to auto-activate conda-forge/miniforge#740

[danyeaw]

Since this was more like a parent Epic with sub issues, I would like to flatten the structure under #1 so that it doesn't appear on the work backlog. So I'm going to close this and additional work will be tracked as part of that issue.