fix trailing slash default/never option for prerendered routes

[dbushell]

Prerendered routes break unless trailing slashes are used:

// routes/+layout.server.js
export const trailingSlash = 'always';

SvelteKit trailingSlash documentation:

This option also affects prerendering. If trailingSlash is always, a route like /about will result in an about/index.html file, otherwise it will create about.html, mirroring static webserver conventions.

If undefined or never prerendered routes like /about fail to serve the /about.html static file.

Deno serveDir responds with 301 redirecting /about to /about/ and does not serve about.html.

[dbushell]

Should be fixed with 33ac97a but kinda messy.