nix: flakify

[nathanshelly]

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

todos

• [ ] use sudo via Touch ID when https://github.com/LnL7/nix-darwin/pull/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

• [ ] https://github.com/nathanshelly/.files/issues/62
• [x] https://github.com/nathanshelly/.files/issues/101
• [x] closes https://github.com/nathanshelly/.files/issues/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