Add --use-git-stash for create-next-app

[SDemonUA]

create-next-app requires empty dir, but it often contains some files already (or it just me?). So, I have to either move files or use git stash manually.

Having flag like --use-git-stash would be really convenient and even more convenient in agentic environment.

It will reduce onboarding friction for devs who experimenting with next.js in mixed env