Add create-zellij-session script

Based on t.nix, this creates or attaches to a Zellij session based on a
given directory.

lib/shared/home-manager-packages.nix

@@ -12,6 +12,9 @@ let
   inherit (pkgs) writeShellApplication;
 
   _timer = writeShellApplication (import ./scripts/_timer.nix);
+  create-zellij-session = writeShellApplication (
+    import ./scripts/create-zellij-session.nix { inherit pkgs; }
+  );
   export-video-list = writeShellApplication (
     import ./scripts/export-video-list.nix { inherit pkgs username; }
   );
@@ -66,6 +69,7 @@ with pkgs;
   yarn
 
   # Scripts.
+  create-zellij-session
   notetaker
   run
   t