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/test1.rs

@@ -7,6 +7,8 @@
 // more than 40 at once, each apple only costs 1! Write a function that calculates
 // the price of an order of apples given the order amount. No hints this time!
 
+// I AM NOT DONE
+
 // Put your function here!
 // fn ..... {