* Add `manifest.webmanifest` and `manifest.(j|t)xs?` support for
manifest.json route
* Add `Manifest` type for it for autocomplete purpose.
Remove the exports for `SitemapFile` and `RobotsFile` globally, will
discuss how to re-export them with better naming later
Small fix for `Robots` typing, should allow `string | string[]` for user
agent of single Robots
Closes NEXT-839
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