try_files for @std/http/file_server

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

there is currently no built-in way to attempt to serve a file from a directory and then handle when the file doesn't exist.

**Describe the solution you'd like**

a function similar to serveDir or serveDir itself but will throw or hand off back in some way to the caller to deal with when a file is missing, similar to the [try_files directive in nginx](https://nginx.org/en/docs/http/ngx_http_core_module.html#try_files).

**Describe alternatives you've considered**

building my own solution but i feel this would be a common enough use case to have as part of the standard library.