Add never return type for redirect() and notFound() (#42009)

Ensures cases like this work:

```
function username(user: null | User) {
  if (!user) redirect();
  return user.name;
}
```
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## Bug

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packages/next/client/components/not-found.ts

@@ -1,6 +1,6 @@
 export const NOT_FOUND_ERROR_CODE = 'NEXT_NOT_FOUND'
 
-export function notFound() {
+export function notFound(): never {
   // eslint-disable-next-line no-throw-literal
   const error = new Error(NOT_FOUND_ERROR_CODE)
   ;(error as any).digest = NOT_FOUND_ERROR_CODE

packages/next/client/components/redirect.ts

@@ -1,6 +1,6 @@
 export const REDIRECT_ERROR_CODE = 'NEXT_REDIRECT'
 
-export function redirect(url: string) {
+export function redirect(url: string): never {
   // eslint-disable-next-line no-throw-literal
   const error = new Error(REDIRECT_ERROR_CODE)
   ;(error as any).digest = REDIRECT_ERROR_CODE + ';' + url