Add a `debug` mode

[wolfv]

It would be nice to have a conda-debug equivalent. It should start by showing you the different variants and a interactive selector. It should also let you select which output you want to debug.

It would then run the build until it hits that output. Then it prepares the environment and stops, prints the directory and then the user can go into the environment and edit it.

[JeanChristopheMorinPerso]

It should also support tests. We should be able to get inside the test environment of our choice.

It might also be a good idea to chose a different name? I always found "debug" not very informative or representative of what the command does. Maybe "--create-env-only"? edit? prepare? Inline?