feat!: change machine hostnames

Instead of merely naming my systems "laptop", "desktop", etc. (boring),
I now have them named after Aristotelian elements (cool).

This also gives me an excuse to refactor my flake file.

machine.nix

@@ -0,0 +1,7 @@
+{ lib, ... }:
+{
+  options.machine = lib.mkOption {
+    description = "The machine hostname to configure for";
+    type = lib.types.enum [ "earth" "fire" "air" "water" ];
+  };
+}