Emit 404.html when nitro preset is *static (prerender)

[yy4382]

Some users (e.g. me) may want to prerender all routes using start, and then deploy the static files. In these cases, a prerendered 404.html is important becuase there are no servers rendering it in runtime.

For example, Nuxt emits 404.html when using its generate or --prerender cli parameters

from nuxt doc
If you deploy your app to static hosting with the nuxi generate or nuxi build --prerender commands, then by default, Nuxt will render every page as a separate static HTML file.

TS start is kind of different, because there is a "fuzzy" 404 handler, but i think it is okay to fallback to "root" if user is using a "*static` nitro preset.

As for when to emit 404.html, I find all *-static (static, vercel-static, github-pages etc.) nitro presets all sets the static opt to true. So simply check server.static in TS start app config (after applying presets) should work.

BTW redirecting everything to / (like in #3144) is a bad idea in this case because if so, the index will flicker for a sec, and, why do I even prerender other routes if I do so?

Edit: Have read SPA Mode Enhancements #3394 carefully. Here are a few arguments that I think _shell.tsx cannot replace a 404.html ouput in this use case (no server, can only use redirect):

1. For netlify, things is kind of easy: just use /* /_shell.html 404. This takes advantage of netlify's shadowing system, by default it will match the real prerendered page, and if there are no matches, it will be redirected to _shell.html, which is a 404 page and returns 404 code
2. But for Vercel, which seems to always shadowing the html file (my tests failed becuase "redirects": [{ "source": "/(.*)", "destination": "/index.html", "statusCode": 200 }] just gives me 404 for all pages) comes two problems:
   1. my prerenders are not used, user will have a slow showing page, and the SEO will be bad;
   2. I CANNOT get a 404 status code for using "redirects": [{ "source": "/[^.]+", "destination": "/", "statusCode": 200 }] becuase if i use 404 in redirect then every route is 404.
3. This does not align with our mental model. We all expected that a static file server finds matched files to serve, and serve a special file when 404. Redirectly all routes to a html which acts as 404, relying on non-shdowing behaviour to match actual files just don't feel right.