feat: improve watch execution mode

The `watch` command now requires user action to move to the next
exercise.

BREAKING CHANGE: this changes the behavior of `watch`.

exercises/functions/functions3.rs

@@ -1,6 +1,8 @@
 // functions3.rs
 // Make me compile! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     call_me();
 }