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
