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/test4.rs

@@ -5,6 +5,8 @@
 
 // Write a macro that passes the test! No hints this time, you can do it!
 
+// I AM NOT DONE
+
 fn main() {
     if my_macro!("world!") != "Hello world!" {
         panic!("Oh no! Wrong output!");