Dynamic routes for toplevel robots and sitemap (#47091)
Support top-level `robots.[ext]` and `sitemap.[ext]` with dynamic api
routes
* Use isAppRoute to determine api routes and metadata routes as metadata
routes are normalized as `<metadata>/route`
* Normalize path to auto append extension to pathname for sitemap.js and
robots.js
* Add typings `SitemapFile` and `RobotsFile`
* move the normalize logic together, reuse the `absolutePathToPage`.
Changes less when touching both dev-server/hot-reloader and next-server,
use the same utils to handle paths
Closes NEXT-262
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