fix(typed-routes): Fix route type fallback (#73271)
### What?
This PR updates the fallback type for typed routes to match the one in
https://github.com/vercel/next.js/blob/canary/packages/next/src/types.ts#L58.
### Why?
During builds, the `Route` type is initialized as the fallback type
before being populated by the routes. This causes temporary type
warnings in the project, since a route like `"" | Route` becomes `"" |
string` which can raise an ts-eslint error like `"" is overridden by
string in this union type`.
This becomes a problem in both failed builds, and in CI where a linter
and a build can run in parallel. Updating the type to match the stub
type fixes the issue.
Co-authored-by: Sam Ko <sam@vercel.com>
Co-authored-by: JJ Kasper <jj@jjsweb.site>
