Refactor Server Router (#39902)

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This is intended to refactor the router code to reduce the overhead of executing routes. This is related to #32314 that may help reduce the memory overhead as this also replaced the `Set` with a `WeakMap`.

Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next/server/dev/next-dev-server.ts

@@ -222,7 +222,6 @@ export default class DevServer extends Server {
       for (const path in exportPathMap) {
         const { page, query = {} } = exportPathMap[path]
 
-        // We use unshift so that we're sure the routes is defined before Next's default routes
         this.router.addFsRoute({
           match: getPathMatch(path),
           type: 'route',