Support viewport export via TS Plugin (#57554)

### What?

The Next.js TS Plugin generally supports the named exports for the pages and layouts.
Currently the TS Plugin doesn't support the new viewport export.

See: https://nextjs.org/docs/app/building-your-application/upgrading/codemods#use-viewport-export

### How?

Added the `viewport` export to the `constants.ts`.

Closes #57680

packages/next/src/server/typescript/constant.ts

@@ -15,6 +15,8 @@ export const ALLOWED_EXPORTS = [
   'generateStaticParams',
   'metadata',
   'generateMetadata',
+  'viewport',
+  'generateViewport',
 ]
 
 export const LEGACY_CONFIG_EXPORT = 'config'