Allow ServeDir to rewrite path to include .html

Feature Request

Motivation

Next.js Static Export results in a /out directory that looks something like

/out/index.html
/out/register.html
/out/404.html

This differs slightly from a traditional SPA where all routes would go to index.html, as a request to /register should use the register.html file.

There may be a way to accomplish what I'm asking, but I couldn't figure it out from the docs and/or examples.
I tried use ServeDir as so

let app = Router::new().nest_service("/", ServeDir::new("out"));

Going to / correctly returns the index page.
Going to /register.html correctly returns the register page.
But I want /register to also return the register page, but it returns 404

Proposal

For a given path without any extension, attempt to append .html. Similar to append_index_html_on_directories, but could have something such as append_html_on_directories where it would only append ".html" rather than "index.html" to the path.

Alternatives

Not sure of any.

Start with the ServeDir service and its append_index_html_on_directories documentation and examples. Determine how an extensionless request should resolve to the corresponding .html file, then verify that /register serves register.html while existing index and explicit .html behavior remains unchanged.