PHP code style proposal

Readable version: https://github.com/tristanlins/contao-standards-recommendation/blob/propose/php-code-style/proposed/csr-x-php-code-style.md
For very long implements, I'd also allow:
```php
<?php
class ClassName
    extends ParentClass
    implements
        \ArrayAccess,
        \Countable,
        \Serializable
{
    // constants, properties, methods
}
```
This is especially handy, when implementing interfaces that have naming clashes like `SerializableInterface` or whatever from different namespaces and therefore would have to be imported via use alias.
This is pretty uncommon, granted, but it will be impossible to solve by maintaining the maximum line length at the same time.