Running `yarn dev` should automatically open the browser

[cinxmo]

Running yarn dev should automatically open the browser

[mythmon]

In my PR for observable login I found that the package open was effective and easy to use for opening browsers:

[mbostock]

I recommend taking a look at how Vite does this:

https://github.com/vitejs/vite/blob/ac2503d52cfc071b5be07485183708dee23b82c5/packages/vite/src/node/server/openBrowser.ts#L23

Like @mythmon says, Vite uses open. I guess we could ignore the niceties that Vite adds… though one feature I particularly like from Vite is that you can type o into the terminal and it opens the browser again.