feature suggestion: `bud run` should launch browser (maybe with a flag to enable?)

When launching the dev server, I have to manually open a browser tab and load the URL. It would be cool if it called out to the OS and simply told it to open the dev server URL. The quick and dirty (on macOS) is to shell out to `open $URL` but I imagine there's a go module somewhere that does this.

Even a flag to do it would be fine, as I invoke `bud run` from a Makefile (`make dev`).

c.f.: https://talk.jekyllrb.com/t/automatically-start-browser-on-localhost-4000-after-jekyll-serve/4916/5

Start by tracing the `bud run` CLI entry point, the dev-server URL construction, and existing flag conventions. Check how the project handles platform-specific process launching before choosing an approach. Done means an opt-in browser launch works for the dev-server URL without changing the default behavior.

Written by the indexing model from the issue text.