feat: add mamba environment

[henryiii]

Closes #444.

Initial prototype, no docs or tests yet. Does this look like a good way to do it?

[henryiii]

Does this look like a good direction? Happy to polish it off if so. Also, shouldn't mamba fall back to conda if mamba is not available? I rather think so?

[henryiii]

@theacodes any direction here on the way you'd like it to go?

[theacodes]

Ah, I'm sorry this kept flying under my radar. This approach is great.

[henryiii]

This implementation is just a simple "mamba" can be chosen implementation - it does not auto-switch to conda if mamba is not available, or install mamba for you. A user could detect this when setting the backend in their noxfile.py, though, if that was important to them, and one of those two things could be added later if commonly used.

[cjolowicz]

Thank you @henryiii

Minor nit for the docs, otherwise this looks good to me! Good call about PassthroughEnv also needing conda_cmd.

[cjolowicz]

Thanks 🚀