add --open to the CLI

[Pomax]

Description

It'd be nice if there was a --open flag that just ran open-cli with the server URL, so folks don't need to either copy-paste URL strings, or set up parallel scripts that have to guess at the URL that needs to be opened.

Why

Because it's one less "tiny bother that adds up over the course of a day's worth of testing" thing folks need to do in order to just focus on their work.

Alternatives

everyone who needs it can manually install open-cli, add it to their npm script, and then run serve and open-cli in parallel. However, the "in parallel" part is not exactly trivial (with npm-run-all being effectively abandoned, and & not being cross-platform compatible syntax) and you'd have to "guess" the server URL. It's probably localhost:3000 but that's configurable so it might not be. Also, looking at the dependency list for serve, adding open-cli to that would barely have an impact on serve's footprint while making dev life just that bit easier.