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/functions4.rs

@@ -4,6 +4,8 @@
 // This store is having a sale where if the price is an even number, you get
 // 10 (money unit) off, but if it's an odd number, it's 3 (money unit) less.
 
+// I AM NOT DONE
+
 fn main() {
     let original_price = 51;
     println!("Your sale price is {}", sale_price(original_price));