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/variables/variables1.rs

@@ -1,6 +1,13 @@
 // variables1.rs
 // Make me compile! Scroll down for hints :)
 
+// About this `I AM NOT DONE` thing:
+// We sometimes encourage you to keep trying things on a given exercise,
+// even after you already figured it out. If you got everything working and
+// feel ready for the next exercise, you the `I AM NOT DONE` comment below.
+
+// I AM NOT DONE
+
 fn main() {
     x = 5;
     println!("x has the value {}", x);

exercises/variables/variables2.rs

@@ -1,6 +1,8 @@
 // variables2.rs
 // Make me compile! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let x;
     if x == 10 {

exercises/variables/variables3.rs

@@ -1,6 +1,8 @@
 // variables3.rs
 // Make me compile! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let x = 3;
     println!("Number {}", x);

exercises/variables/variables4.rs

@@ -1,6 +1,8 @@
 // variables4.rs
 // Make me compile! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let x: i32;
     println!("Number {}", x);