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/structs/structs1.rs

@@ -1,6 +1,8 @@
 // structs1.rs
 // Address all the TODOs to make the tests pass!
 
+// I AM NOT DONE
+
 struct ColorClassicStruct {
     // TODO: Something goes here
 }

exercises/structs/structs2.rs

@@ -2,6 +2,8 @@
 // Address all the TODOs to make the tests pass!
 // No hints, just do it!
 
+// I AM NOT DONE
+
 #[derive(Debug)]
 struct Order {
     name: String,