Cloudflare Pages: Don’t overwrite _routes.json if present in the public folder

[AWBroch]

Describe the feature

Auto-generating a _routes.json file for Cloudflare Pages is great, but there are some cases where it makes sense to just manually write a _routes.json
For example, if you have more than 100 static routes, or if an API path that is available during pre-rendering isn’t available on the server (for example, IPX/nuxt image)
Before #965 I could just put a _routes.json in my public folder and it would be copied and deployed, but after #965 it would overwrite my manual _routes.json with the auto-generated one. So, it would be nice if Nitro didn’t overwrite manual _routes.json files.
I’ve prepared a fix that passes all the tests and I can open a PR if this sounds good.

Additional information

• Would you be willing to help implement this feature?