Automatically restart the preview server when the config file changes

[mbostock]

Maybe we can do something using tsx watch in the default template so that the server restarts when the config file is edited? (You’d still need to reload in your browser, but maybe #50 would help with that.)

[mbostock]

I think this is probably best addressed by making the config lazy as in #695; that way we don’t need to restart the server, but we’ll always get the latest config on request. We could also watch the config file and force a reload, perhaps.