This adapts the new client hooks of `usePathname`, `useSearchParams`,
and `useRouter` to work within the `pages/` directory to aid users
attempting to migrate shared components over to the `app/` directory.
> **Exception:**
> When the pages router is not ready, `useSearchParams` will return an
empty `URLSearchParams`. This mirrors the behavior seen in the `pages/`
directory today in that `router.query` is not available until the client
hydrates.
This also adds a new option for `useRouter` to bring it line with the
correct typings with the app directory. By default, calling
`useRouter()` will return the type `NextRouter | null` to represent what
you get when you call it from a component originating from the app
directory. If you want to instead force it to return `NextRouter` as it
does today, you can pass a boolean into the `useRouter` call as such:
```ts
const router = useRouter() // typeof router === NextRouter | null
const router = useRouter(true) // typeof router === NextRouter
```
This change is designed to ease the incremental adoption of app.
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