Provide www redirect

Hello,

It'd be nice if Deno Deploy had some kind or support / documentation / feature (like Netlify) for www redirects when adding a custom domain.

For example, configuring a site with a custom domain, I'd like to be able to 301 redirect non-www requests to www.

https://example.com > https://www.example.com

No repository files, tests, or entry points are mentioned. Start by reviewing Deno Deploy's custom-domain configuration and existing redirect support; done means providing a documented or configurable 301 redirect from https://example.com to https://www.example.com.

Written by the indexing model from the issue text.