Lift public router instance to module level (#77426)
There's only one instance for the whole app, and it conceptually lives
outside of the React tree. We don't need to create it during render.
The intent here is also to provide a clearer separation between the
public router interface (returned by `useRouter`) and internal routing
implementation used by Next.js itself, e.g. in the Link component. Right
now, most internals go through the public interface, but going forward
we should call the lower level methods directly. This helps prevent
implementation details from accidentally leaking into public APIs.
