[PHP] Implicit implements Stringable declaration should produce warning

### Apache NetBeans version

Apache NetBeans 18

### What happened

Stringable is implicitly present on any class that has the magic ```__toString()``` method defined. It is considered a bad habit not to explicitly state a class as Stringable while it has ```__toString()```.

### How to reproduce

```php

Reproduce the example in Apache NetBeans 18 with a class defining __toString() without explicitly implementing Stringable, and compare it with the explicitly implemented class. Trace the PHP diagnostics for this case and verify that the implicit declaration produces a warning while the explicit implementation remains accepted.

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