Add a native fish Module and set it as neogaia's login shell

Translate the CachyOS fish snapshot into a home-manager Module: the
fastfetch greeting, the bat-backed manpager, the done and bang-bang
plugins (from nixpkgs, not a plugin manager), the history/backup/copy
helper functions, and the eza and navigation aliases. Pacman-specific
aliases are dropped or replaced with NixOS equivalents (update ->
nixos-rebuild switch, cleanup -> nix-collect-garbage). Enabling the
Module also makes fish the user's default login shell.

hosts/neogaia/default.nix

@@ -35,6 +35,9 @@
   # An SSH daemon so the rest of the setup can be driven over the network.
   services.openssh.enable = true;
 
+  # fish as the login shell.
+  modules.fish.enable = true;
+
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";