async stop() {
        if (this.state.eventSource)
            this.state.eventSource.close();
        if (!this.state.httpServer)
            return;
        const server = this.state.httpServer;
        return new Promise((resolve) => server.close(resolve));
    }
Add a prepared subcommand.

See .command() for creating an attached subcommand which inherits settings from its parent.

@param {Command} cmd - new subcommand
@param {Object} [opts] - configuration options
@return {Command} `this` command for chaining
@api public