nix: flakify

[nathanshelly]

v2.0 of #105 - still very much a work in progress

todos

• use sudo via Touch ID when Add option to enable sudo authentication with Touch ID LnL7/nix-darwin#228 is merged
  • will need to set up pam_reattach to be able to use inside tmux
• come up with migration story from previous version of dotfiles
• conduct more detailed tests
  • on new machine
  • upgrading from previous config
  • in CI
• update root README.md - at the very least the "getting started" section
• add CI step to extract configuration files for anyone who wants to use parts of this config w/o Nix
• investigate https://github.com/haslersn/any-nix-shell
  • investigate shell completions
• use mkOutOfStoreSymlink to symlink mutable files
• define aliases via home manager

related issues that can be cleaned up

• Add setup flag to set up complete (quirks & all) config #62
• Manage ssh config via Nix #101
• closes Manage vim plugins via nix #104

[nathanshelly]

should switch to the new 2.4 release when it comes out then merge this in and continue building on top of it

a few resources to read:

• https://nixos.wiki/wiki/Flakes#Output_schema
• https://github.com/NixOS/nix/pull/5308/files
• https://discourse.nixos.org/t/difference-between-nix-shell-and-nix-run/2951/18
• https://nixos.wiki/wiki/Nix_command
• https://nixos.org/manual/nix/unstable/command-ref/experimental-commands.html