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

**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() }`