home: wm: add i3

This is only the basic default configuration. The full configuration
will come later.

I am not a fan of `wm.windowManager`, I might rename that option at some
point.

home/wm/default.nix

@@ -0,0 +1,15 @@
+{ lib, ... }:
+{
+  imports = [
+    ./i3.nix
+  ];
+
+  options.my.home.wm = with lib; {
+    windowManager = mkOption {
+      type = with types; nullOr (enum [ "i3" ]);
+      default = null;
+      example = "i3";
+      description = "Which window manager to use for home session";
+    };
+  };
+}