Fix PHPStan detected errors

I'm attaching the initial report with all the issues detected before fixing anything:

Hello,

I've created an initial MR fixing most of the errors reported. It still pending the following one:

 ------ --------------------------------------------------------------------- 
  Line   modules/file_example/src/FileExampleSubmitHandlerHelper.php          
 ------ --------------------------------------------------------------------- 
  460    Class Drupal\devel\DevelDumperInterface not found.                   
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols  

I don't really know if by using Stub files we can fix it since it is a conditional class not always present (depending is Devel is required or not).

I've checked in deep the error from #5 but I think Stub files is not the right way because they are meant for other kind of scenarios according to (https://phpstan.org/user-guide/stub-files). Therefore I don't see other way to get rid of it than ignoring it via php-baseline.neon

I've added a new commit with this approach.

Now we are passing the PHPStan checks for Level 1:

$ php ../vendor/bin/phpstan analyze --configuration modules/contrib/examples/phpstan.neon modules/contrib/examples
 242/242 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%
                                                                                                                        
 [OK] No errors                                                                                                      

I think this is now ready to be reviewed.

After [#] has been merged, now we can see we are passing the PHPStan check within !32 if we compare it with the last run on the main project (https://git.drupalcode.org/project/examples/-/pipelines/85372)

Therefore I'd say this is fully ready to be reviewed by someone else.

Thanks!