Fixes #485

This is the simplest approach I could think of--I know it can get a little hairy with all the versions of Svelte and SvelteKit out there. Still, I believe most of the logic is already in place with the CLI to make sure the user has the right versioning heading into the migration.

This only runs the app-state migration script if the user has SvelteKit. It doesn't really log anything or add any error handling--I'll leave those up to you if desired.

Also, I know you guys have a contributing guide on the docket, but I was wondering how exactly you test this CLI on your own projects. I ask because I ended up using npm link which is pretty standard, but I had to edit these lines of code to make sure that when I tested my fixes locally that it linked to my scripts instead of pulling @latest from npx 🙏