feat!(flake): convert to fs-defined configurations

Place all configurations in `machines/${manager}/${system}/${hostname}/...`:
    - `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
    - `${system}` - A system's architecture (can be many)
    - `${hostname}` - A system's hostname (can be many)

The flake now automatically generates the needed configurations based on the above structure.

It only generates configurations for valid directory structures:
    - It wouldn't generate a NixOS config if there isn't a `configuration.nix` file
    - ...

machines/nix-darwin/x86_64-darwin/apavel-a01/configuration.nix

@@ -0,0 +1,32 @@
+{ inputs, outputs, lib, config, pkgs, ... }:
+
+{
+  environment.systemPackages = with pkgs; [];
+
+  # environment.darwinConfig = builtins.toString ./configuration.nix;
+
+  # Auto upgrade nix package and the daemon service.
+  services.nix-daemon.enable = true;
+
+  nix.package = pkgs.nix;
+
+  # Create /etc/zshrc that loads the nix-darwin environment.
+  programs.zsh.enable = true;  # default shell on catalina
+
+  # Fonts
+  fonts.fontDir.enable = true;
+  fonts.fonts = with pkgs; [
+    (nerdfonts.override { fonts = [ "FiraCode" ]; })
+  ];
+
+  # Keyboard
+  system.keyboard.enableKeyMapping = true;
+  system.keyboard.remapCapsLockToEscape = true;
+
+  # Add ability to used TouchID for sudo authentication
+  security.pam.enableSudoTouchIdAuth = true;
+
+  # Used for backwards compatibility, please read the changelog before changing.
+  # > darwin-rebuild changelog
+  system.stateVersion = 4;
+}