Hotfix - FQCN in `extends` and `implements`

This PR is a BC break. 

Similar to #4149, it is currently problematic to generate multiple classes into the same file. 

This PR introduces fully qualified class names for the `extends` and `implements` clauses in generated code.

Before

```php
class Foo extends Bar implements Baz {}
```

After

```php
class Foo extends \Bar implements \Baz {}
```