Calling `aspire new` on repo with existing files fails

Calling `aspire new` with a single-file app on a project that already contains files fails. We should probably have some sort of `--force` flag here?

```
$ aspire new
Enter the project name (aspire-11474): aspire-11474
Enter the output path: (./aspire-11474): .
📦 Using project templates version: 9.5.0-preview.1.25468.13
❌ The new project cannot be created because it would overwrite existing files in the output folder. Please remove or move the conflicting files, or choose a different location.
```