Rewrite for cross-platform compatibility (following conda/ceps#8)

[jaimergp]

CEP: conda/ceps#8

To do

• Create new project scaffold and move old code to _legacy
• Create JSON-Schema for the input files
• Implement shortcut creation for Windows
• Implement shortcut creation for Linux
• Implement shortcut creation for MacOS
• Implement environment activation on Unix
• Implement environment activation on Windows
• Cleanup tasks: conda [env] remove should also delete created menu items
• Do not depend on pydantic at runtime
• Implement superuser elevation on Windows
• Implement superuser elevation on Linux (?)
• Implement superuser elevation on MacOS (?)
• Test elevation
• Handle interference of PYTHON* variables with shortcuts
• macOS integrations: entitlements and permissions
• macOS integrations: keyboard shortcuts registration
• Clean up unused code
• Sort common Windows utilities

[hoechenberger]

Wow. All but one checkboxes of the todo are ticked ✅ Does this mean this work is almost done? 😲😎

[jaimergp]

Not yet, I am debugging some elevation stuff I am not happy with, and we also got a report where PYTHON* variables disrupt the launch of the program.

But it's getting closer :)

[hoechenberger]

Nice, thanks for the hard work!!

[jaimergp]

@conda/constructor - how do you feel about merging this to a feature branch to keep further iterations well isolated?

[jezdez]

@jaimergp +1

For sanity reasons, I would also suggest polishing the CEP and get it voted on ASAP.

[jaimergp]

Updated the target branch and merging.