Attribute completion: restrict result list by target, if present

Based on  https://www.php.net/manual/en/language.attributes.classes.php we have 
```
    Attribute::TARGET_CLASS
    Attribute::TARGET_FUNCTION
    Attribute::TARGET_METHOD
    Attribute::TARGET_PROPERTY
    Attribute::TARGET_CLASS_CONSTANT
    Attribute::TARGET_PARAMETER
    Attribute::TARGET_ALL
```
