feat: move self-defined packages to own files

pkgs/timer/default.nix

@@ -0,0 +1,9 @@
+{ writeShellApplication, speechd }:
+
+writeShellApplication {
+  name = "timer";
+  runtimeInputs = [ speechd ];
+  text = ''
+    sleep "$1"; while true; do spd-say "$2"; sleep 0.5; done;
+  '';
+}