Add support for an SPA fallback in `@std/http/file-server`

[lishaduck]

Is your feature request related to a problem? Please describe.

I'm deploying an SPA that uses path routing, so I need to convert 404s into requests for the index.html page.

Describe the solution you'd like

Add in support for a fallback page to fileServer

Describe alternatives you've considered

if (res.status == 404) { serveFile() }

[kt3k]

probably duplicate of #3533 ?

[lishaduck]

probably duplicate of #3533 ?

Probably, assuming it wouldn't send it as a 404? I'd need it to be a 200.

[kt3k]

Ah, ok. Then this would be a bit different feature request.