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/move_semantics/move_semantics1.rs

@@ -1,6 +1,8 @@
 // move_semantics1.rs
 // Make me compile! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let vec0 = Vec::new();
 

exercises/move_semantics/move_semantics2.rs

@@ -1,6 +1,8 @@
 // move_semantics2.rs
 // Make me compile without changing line 10! Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let vec0 = Vec::new();
 

exercises/move_semantics/move_semantics3.rs

@@ -3,6 +3,8 @@
 // (no lines with multiple semicolons necessary!)
 // Scroll down for hints :)
 
+// I AM NOT DONE
+
 fn main() {
     let vec0 = Vec::new();
 

exercises/move_semantics/move_semantics4.rs

@@ -3,6 +3,8 @@
 // in `fn main`, we instead create it within `fn fill_vec` and transfer the
 // freshly created vector from fill_vec to its caller. Scroll for hints!
 
+// I AM NOT DONE
+
 fn main() {
     let vec0 = Vec::new();