fix: add back rootId environment option"

This reverts commit dead5e3.

Author: danielroe

src/config.ts

@@ -192,6 +192,11 @@ declare module 'vitest' {
        */
       url?: string
       overrides?: NuxtConfig
+      /**
+       * The id of the root div to which the app should be mounted. You should also set `app.rootId` to the same value.
+       * @default {nuxt-test}
+       */
+      rootId?: string
       /**
        * The name of the DOM environment to use.
        *