Don't fail if directory already exists

[aaronpowell]

If you set . as the directory to install keystone into, it'll fail on https://github.com/keystonejs/create-keystone-app/blob/main/create-keystone-app/src/index.ts#L84

It'd be better to check if the directory exists, and if it does, warn that you might override existing files.

I hit this often as the workflow I use is to make the folder, open VS Code and then do the setup from there.