Beginnings of config for Darwin machines

Split out a platforms/ directory with a linux.nix and darwin.nix, and
starting splitting things out between the two

home/platforms/darwin.nix

@@ -0,0 +1,24 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  home.packages = with pkgs; [
+    coreutils
+    gnupg
+    pinentry_mac
+  ];
+
+  home.activation.linkApplications = lib.hm.dag.entryAfter ["writeBoundary"] ''
+    $DRY_RUN_CMD ln -sf $VERBOSE_ARG \
+      ~/.nix-profile/Applications/* ~/Applications/
+  '';
+
+  programs.zsh.initExtra = ''
+    export NIX_PATH=$HOME/.nix-defexpr/channels:$NIX_PATH
+
+    if [[ "$TERM" == "alacritty" ]]; then
+      export TERM="xterm-256color"
+    fi
+  '';
+}