chore(flake): add plasma-manager input

nix-community/plasma-manager provides declarative KDE Plasma
configuration through Home Manager. Follows our nixpkgs and
home-manager.

Author: khaneliman

flake.lock

@@ -119,6 +119,13 @@
       url = "github:Mic92/fast-nix-gc";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    plasma-manager = {
+      url = "github:nix-community/plasma-manager";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+        home-manager.follows = "home-manager";
+      };
+    };
     stylix = {
       url = "github:danth/stylix";
       inputs = {