Tunnel / reverse proxy for CLI to enable sharing across the web

## Feature Request

Tools like ngrok and wrangler provide a reverse proxy / tunnel that lets you plumb your local application out into the world. We already expose apps on the user's network, but we could also allow apps to be exposed on the global internet for testing purposes.

Instead of deploying an app, you'd simply turn on the tunnel feature of the CLI and your app would be available to share in slack/github/etc.

Cloudflare provides free tunnels via cloudflared which (ToS permitting...) we can simply embed in the CLI.

https://developers.cloudflare.com/cloudflare-one/connections/connect-networks/do-more-with-tunnels/trycloudflare/

There's an open source version we can host on a tiny vps.

https://github.com/ekzhang/bore