Add countdown example

examples/cli/.gitignore

@@ -1,2 +1,3 @@
+countdown
 echo
 form

examples/cli/countdown.roc

@@ -0,0 +1,18 @@
+app "countdown"
+    packages { pf: "platform" }
+    imports [ pf.Stdin, pf.Stdout, pf.Task.{ await, loop, succeed } ]
+    provides [ main ] to pf
+
+main =
+    _ <- await (Stdout.line "\nLet's count down from 10 together - all you have to do is press <ENTER>.")
+    _ <- await Stdin.line
+    loop 10 tick
+
+tick = \n ->
+    if n == 0 then
+        _ <- await (Stdout.line "🎉 SURPRISE! Happy Birthday! 🎂")
+        succeed (Done {})
+    else
+        _ <- await (n |> Num.toStr |> \s -> "\(s)..." |> Stdout.line)
+        _ <- await Stdin.line
+        succeed (Step (n - 1))