Watch recompiles all the files each time

Hey 👋 there!

Thanks for this awesome project, it's really helpful for beginners (like me). I'd like to suggest an improvement (and probably would be happy to implement).

When I run `rustlings watch` my learning flow is simple: walk through all files one by one and fix errors. Does it make sense **to verify only changed files** (most like only the last one), not the whole set?

Thanks! 👍 