feat(dotfiles): Begin controlling various dotfiles from Nix

Moves my i3 configuration into a nix derivation called 'tazjins-dotfiles'.

A good step towards fully declarative system configuration!

dotfiles.nix

@@ -0,0 +1,31 @@
+# Bundle configuration files into a derivation.
+# I call this derivation dotfiles despite that not technically being true
+# anymore ...
+
+{ config, pkgs, ...}:
+
+let dotfiles = pkgs.stdenv.mkDerivation {
+  name = "tazjins-dotfiles";
+
+  srcs = [
+    ./dotfiles
+  ];
+
+  installPhase = ''
+    mkdir -p $out/dotfiles
+    cp ./* $out/dotfiles/
+  '';
+};
+in {
+  # /etc/ is a special place in NixOS!
+  # Symlinks that need to be created there must be specified explicitly.
+  environment.etc = {
+    "i3/config" = {
+      source = "${dotfiles}/dotfiles/i3.conf";
+      # Setting a mode causes Nix to copy the file instead of symlinking it.
+      # For i3 config in particular this is desirable because I want to be able
+      # to modify and reload it before committing a change.
+      mode = "0644";
+    };
+  };
+}