PHPstan integration

[rikvdh]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Since I need to move away from ext-imap and also from laminas/mail I'm evaluating options for IMAP interaction in PHP.

For our project we want to serve a high quality level. I was testing this library against phpstan (static analyzer). I found a lot of potential problems. I was working on some of them already. But before continuing my work I want to discuss this as this will come all together as a big change.

See my current change-set:
https://github.com/Webklex/php-imap/compare/master...rikvdh:php-imap:feature/phpstan-integration?expand=1

(I'm not done yet)

Describe alternatives you've considered
Reason to move away from ext-imap:
php/pecl-mail-imap#15

Reason to move away from laminal/mail:
laminas/laminas-mail#269

[rikvdh]

@Webklex Is phpstan support something you want integrated? Then I can continue my work on this.