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/i3.nix

@@ -0,0 +1,15 @@
+{ config, lib, ... }:
+let
+  isEnabled = config.my.home.wm.windowManager == "i3";
+in
+{
+  config = lib.mkIf isEnabled {
+    xsession.windowManager.i3 = {
+      enable = true;
+
+      config = {
+        modifier = "Mod4"; # `Super` key
+      };
+    };
+  };
+}