Add test for example module

I am done with my changes. Please review. I made the following changes:

* Fixed Drupal coding standards and Drupal practices
* Implemented static code analysis via PHPStan and PHPCS
* Added testing via PHPUnit. The test proves if a new form field is attached via form_decorator_example with the API that form_decorator provides
* Spellchecker via cspell

Yayy tests are running :D

Thank you!

Can we separate the tests from the cleanups? Because I'd merge the tests right away.

The static code analysis is also tests. It's tests without explicit written tests. That way the Drupal coding standards are proven. You are loosing nothing here. I would like to ask you kindly about merging.

Ah, I've seen your notices for the merge request. Let me check them.

@harlor: I have answered on your notices. Could you please review again?

@harlor: I resolved all your notices. Please review.

@harlor: Thanks for your hints and the suggested changes. 👍 I've tested the module manually on a local Drupal playground site and there were no errors. The tests are all green. Please review.

Unfortunately I still got some "must be compatible with..." Fatals

Thanks again for testing. I feel that we are close. :)

I've resolved your notices. The return type declarations are reverted to the previous state, where they are not causing errors. Please test again.

Thank you so much!

Nice! 💪🎉🥳

Automatically closed - issue fixed for 2 weeks with no activity.