Revisit uninstallers

[jaimergp]

Checklist

• [X] I added a descriptive title
• [X] I searched open requests and couldn't find a duplicate

What is the idea?

Research the best way to implement uninstallers. Currently, only Windows provides some, and they do not do all tasks in the conda-canonical way:

• Files are batch removed without running pre-unlink logic
• Shortcuts are removed by manually calling some menuinst logic
• Windows Registry does not use conda init --reverse (#572)
• I am not sure if the .condarc files are cleared up (related to #568)
• Environments are not removed from ~/.conda/environments.txt

The uninstallation logic is not present on Linux or macOS either.

Why is this needed?

Better uninstallation processes.

What should happen?

The system should be left in a functional state, with no lingering files or configuration.

Additional Context

Uninstallation times have been previously reported as slow, so we might need to balance speed and correctness. See https://github.com/conda/constructor/issues/168. It's mostly UI related, so as long as the output is kept in check, we should be ok. Nevertheless, timings should be analyzed.