By popular request, this pull request adds support for returning `fallback: 'blocking'` from `getStaticPaths`.

This new mode will cause unknown paths to be rendered on-demand ("SSR") without the static (placeholder) fallback.

This feature is **currently experimental and should not be used in production yet**. It's currently flagged behind `unstable_`:

```
fallback: 'unstable_blocking'
```

TODO:

- [x] Next.js tests
- [ ] Add Vercel support
- [ ] Vercel tests

---

Fixes #15637