Enable option for index.html to be a directories default

[ukd1]

Description of the Change

Sometimes (often?) when people serve static files, they may want "pretty urls" - e.g. "/welcome" vs "/welcome/index.html". This config option enables index.html to be the default if the request is for a folder and the file exists, otherwise it defaults to directory listing (if enabled).

Alternate Designs

I considered just doing it in my app directly, but I belive this is a useful & missing minor feature.

Benefits

Serving static html from pretty folders is now easy.

Possible Drawbacks

Except for the additional code, none I can think of as it's disabled by default. I'm acutally not sure where to document this though.

[sdogruyol]

Thanks a lot @ukd1 👍 I think this is a valid use-case and maybe in the future we can make this the default behavio. Also we can document this at the Kemal guide https://github.com/kemalcr/kemalcr.com