Added phpdoc_separation fixer

The idea here is to go from:

```php
    /**
     * @param EngineInterface $templating
     * @return void
     */
```

to 

```php
    /**
     * @param EngineInterface $templating
     *
     * @return void
     */
```
Sure, thanks. This fixer is quite incomplete at the moment. For example, it currently is failing with multiline `@param` descriptions. :(