Add a launcher start method and event to access the Window object

A new method and event should be added when the launcher starts so that the Window object can be accessed. At the moment, editor.call("launcher:start", launcher) is triggered here, but a clearer and more convenient API is needed to work with the opened window.

        const features = popup ? 'popup' : undefined;
        const launcher = window.open('', '_blank', features);
        if (launcher) {
            launcher.opener = null;
            launcher.location = url;
            editor.call("launcher:start", launcher); // Fire event
        }

Start at the launcher startup path shown in the issue, where window.open creates the launcher and editor.call("launcher:start", launcher) is triggered. Trace how launcher methods and events are exposed, then add the requested start method and event so the opened Window object is accessible through the clearer API. Done means the API is available when the launcher starts.