Renamed plasma and niri config files to align with "default.nix" directory standard

modules/default.nix

@@ -4,8 +4,8 @@
     ./configuration.nix
 
     #./hardware-configuration.nix
-    ../plasma/plasma.nix
+    ../plasma
-    ../niri/niri.nix
+    ../niri
   ];
 
   specialisation = {
@@ -13,7 +13,7 @@
       inheritParentConfig = false;
       configuration = {
         imports = [
-          ../plasma/plasma.nix
+          ../plasma
           ./configuration.nix
         ];
 
@@ -36,7 +36,7 @@
       inheritParentConfig = false;
       configuration = {
         imports = [
-          ../niri/niri.nix
+          ../niri
           ./configuration.nix
         ];