Stubs missing for `RustClosure`

Functions returning a `RustClosure` have stubs generated with

```php
    function test_closure(): \RustClosure {}
```

`RustClosure` is missing stubs though.

Possible solutions:

- Include that stub with the module stub if `RustClosure` is used
- Place this in a seperate file if `RustClosure` is used
- Provide a stub file with this crate containing that stub.