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/test2.rs

@@ -7,6 +7,8 @@
 // you think each value is. That is, add either `string_slice` or `string`
 // before the parentheses on each line. If you're right, it will compile!
 
+// I AM NOT DONE
+
 fn string_slice(arg: &str) {
     println!("{}", arg);
 }