Problem: It's possible to use 'deploy-rs' for deploying 'darwinSystem'
configuration from 'nix-darwin' to a darwin system. However, there is no
dedicated activatiot script for darwin and thus one has to come up with
'custom' activation script.
Solution:
1) Add 'darwin' attribute to 'lib.activate' that provides a script that
should be used to activate 'darwinSystem' config with 'deploy-rs'.
2) Add a new 'examples/darwin' example that provides simple flake for
deploying configuration to a darwin target.
