Use greetd and regreet instead of lightdm

LightDM brings problems with setting up the session which leads to
kanshi, swayidle and such services to not get the correct environment
variables like `WAYLAND_DISPLAY`.

modules/boot-login.nix

@@ -16,10 +16,10 @@
     displayManager.defaultSession = "sway";
     xserver.displayManager = {
       lightdm = {
-        enable = true;
+        enable = false;
         greeters.gtk.enable = false;
         greeters.gtk-custom = {
-          enable = true;
+          enable = false;
           theme = {
             name = "Adwaita-dark";
           };
@@ -32,4 +32,22 @@
     };
   };
 
+  services.greetd = {
+    enable = true;
+    settings = {
+      skip_selection = true;
+    };
+  };
+
+  programs.regreet = {
+    enable = true;
+    theme = {
+      name = "Adwaita-dark";
+    };
+    iconTheme = {
+      name = "Arc";
+      package = pkgs.arc-icon-theme;
+    };
+  };
+
 }