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() }