Fallback handler to requests outside of baseURL

[pi0]

When a custom baseURL is set, it is expected that all routes (and static assets) will be prefixed with {baseURL}/ so one should go to https://<addr>/{baseURL}.

If a request hits https://<addr>/* outside of {baseURL}, nitro renders a 404 because there is no route.

#2713 proposed a DX improvement to auto redirect /* to /{baseURL}* (only in dev) however it causes issues (#2881) as there are valid cases that a middleware or proxy route rule might actually need to handle this fallback.

In summary: This a nice DX but has downside of limiting some advanced cases and also potentially more mismatch behavior between dev and prod runtime behavior.

I am thinking of some progressive steps to handle:

• Improve dev error to show a hint that tells users it is 404 but hey, you might want need to go to this link.
• Add an opt-in option to enable baseURL redirect (in two modes: fallback|enforce)
  • we should keep it opt-in for v2, we might enable by default for 3.0 however I'm not sure if trade-off worth it (if we do, we need clear docs about behavior and why/how opt-out)