Expose the generated Vercel deployment url even when using the alias domains option

Currently, if the `alias-domains` param is used, the `preview-url` output is set to the first of the alias domains. As a result, there is no way to access the [generated url](https://vercel.com/docs/deployments/generated-urls#url-with-vercel-cli) for the deployment.

Sometimes it's useful to have this url for other operations after the deployment has completed. It would be great to have it available as an output of the action (eg as `deployment-url`).