create forked impl for Link component (#73019)
There's significant drift between how Link is implemented in pages vs app dir, and we're planning to make even more changes. (For example, we want to change the intersection observer prefetch behavior, but only in app router)
To simplify this drift in behavior, this will alias `next/link` to `client/app-dir/link.tsx` in app dir bundles.
There is no change in behavior to the `link` component in this PR. The next PR in the stack will remove the current forked handling to remove things that are irrelevant to the router it's rendered in.
