Fix PHPStan

Problem/Motivation

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  Line   modules/custom/select_icons/select_icons.module                                                                    
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  9      Function select_icons_theme() has no return type specified.                                                        
  9      Function select_icons_theme() has parameter $existing with no type specified.                                      
  9      Function select_icons_theme() has parameter $path with no type specified.                                          
  9      Function select_icons_theme() has parameter $theme with no type specified.                                         
  9      Function select_icons_theme() has parameter $type with no type specified.                                          
  32     Function template_preprocess_select_icons() has no return type specified.                                          
  32     Function template_preprocess_select_icons() has parameter $variables with no type specified.                       
  32     PHPDoc tag @param has invalid value ($variables                                                                    
           An associative array containing:                                                                                 
           - element: An associative array containing the properties of the element.                                        
             Properties used: #title, #value, #options, #description, #extra,                                               
             #multiple, #required, #name, #attributes, #size.): Unexpected token "$variables", expected type at offset 348  
  50     Function select_icons_library_info_alter() has no return type specified.                                           
  50     Function select_icons_library_info_alter() has parameter $extension with no type specified.                        
  50     Function select_icons_library_info_alter() has parameter $libraries with no type specified.                        
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  Line   modules/custom/select_icons/tests/modules/select_icons_test/src/Form/SelectIconsFormElementTestForm.php          
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  43     Method Drupal\select_icons_test\Form\SelectIconsFormElementTestForm::submitForm() has no return type specified.  
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  Line   modules/custom/select_icons/tests/src/FunctionalJavascript/SelectIconsElementIntegrationTest.php  
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         Class Drupal\FunctionalJavascriptTests\JavascriptTestBase not found.                              
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols                               
  12     Class Drupal\FunctionalJavascriptTests\JavascriptTestBase not found.                              
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols                               
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