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/strings/strings1.rs

@@ -1,6 +1,8 @@
 // strings1.rs
 // Make me compile without changing the function signature! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let answer = current_favorite_color();
     println!("My current favorite color is {}", answer);

exercises/strings/strings2.rs

@@ -1,6 +1,8 @@
 // strings2.rs
 // Make me compile without changing the function signature! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let word = String::from("green"); // Try not changing this line :)
     if is_a_color_word(word) {