feat(schema): add experimental multiApp flag (nuxt#27291)

Author: nicolaspayot

packages/schema/src/config/experimental.ts

@@ -45,6 +45,11 @@ export default defineUntypedSchema({
      * @type {3 | 4}
      */
     compatibilityVersion: 3,
+    /**
+     * This enables early access to the experimental multi-app support.
+     * @see [Nuxt Issue #21635](https://github.com/nuxt/nuxt/issues/21635)
+     */
+    multiApp: false,
     /**
      * This enables 'Bundler' module resolution mode for TypeScript, which is the recommended setting
      * for frameworks like Nuxt and Vite.