Add timer script for desktop machines

Based on https://github.com/sdaschner/dotfiles/blob/master/bin/timer and
https://github.com/sdaschner/dotfiles/blob/master/bin/_timer.

Seen in https://youtu.be/8CZIv3MJvic?si=PiVPM_vwuUKI7ak3&t=1460.

lib/shared/scripts/timer.nix

@@ -0,0 +1,20 @@
+{
+  name = "timer";
+
+  runtimeInputs = [ ];
+
+  text = ''
+    if [[ "$1" == "" ]]; then
+      echo "Usage: ''${0##*/} <mins> [message]"
+      exit 1
+    fi
+
+    mins=$1
+    message=$2
+
+    nohup _timer "$mins" "$message" &> /dev/null &
+
+    echo "timer started for $mins min"
+    echo "timer started for $mins min, message: '$message'" | systemd-cat -t timer
+  '';
+}