Improve NextApiHandler type for early returns (vercel#35166)

I would like to be able to write handlers with early returns like this:

```typescript
import { NextApiHandler } from 'next'

const handler: NextApiHandler = (req, res) => {
  const value = getStuff()
  if (value === 'branch') {
    return res.json({}) 
  }
  res.status(400)
}
```

but `NextApiHandler`'s current return type is `void | Promise<void>`, which causes compilation to fail with

```
Error:(11, 3) TS2322: Type '(req: NextApiRequest, res: NextApiResponse<any>) => Promise<NextApiResponse<any> | undefined>' is not assignable to type 'NextApiHandler<any>'.
  Type 'Promise<NextApiResponse<any> | undefined>' is not assignable to type 'void | Promise<void>'.
    Type 'Promise<NextApiResponse<any> | undefined>' is not assignable to type 'Promise<void>'.
      Type 'NextApiResponse<any> | undefined' is not assignable to type 'void'.
        Type 'NextApiResponse<any>' is not assignable to type 'void'.
```
to avoid that the above snippet needs to be written as

```typescript
  if (value === 'branch') {
    res.json({}) 
    return
  }
```
which looks odd to me. Changing the return type of `NextApiHandler` to `unknown | Promise<unknown>` would allow for shorter early returns and still communicates to users that nothing is expected to be returned from a handler.

Augmenting the type like this, makes the first snippet work:
```typescript
import { NextApiRequest, NextApiResponse } from 'next'

declare module 'next' {
  export declare type NextApiHandler<T = any> = (
    req: NextApiRequest,
    res: NextApiResponse<T>
  ) => unknown | Promise<unknown>
}
```



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

packages/next/shared/lib/utils.ts

@@ -267,7 +267,7 @@ export type NextApiResponse<T = any> = ServerResponse & {
 export type NextApiHandler<T = any> = (
   req: NextApiRequest,
   res: NextApiResponse<T>
-) => void | Promise<void>
+) => unknown | Promise<unknown>
 
 /**
  * Utils