next.js

54fce53d - Make sure the global not found route doesn't conflict with existing /not-found route (#47619)

Make sure the global not found route doesn't conflict with existing /not-found route (#47619) In #47328 we made the root level `/not-found.js` a special entry, to override the page 404 during builds. However, it's possible that the user has a valid `/not-found/page.js` route that might conflict with this special entry. This PR changes the entry to be `/_not-found` so it will never conflict with existing valid entries.