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/tests/tests3.rs

@@ -3,6 +3,8 @@
 // the test passes. Then write a second test that tests whether we get the result
 // we expect to get when we call `is_even(5)`. Scroll down for hints!
 
+// I AM NOT DONE
+
 pub fn is_even(num: i32) -> bool {
     num % 2 == 0
 }