Fix code quality and best practices

Problem/Motivation

- composer.json is not needed as the module as no special dependencies.
- LICENSE.txt is added automatically by Drupal.org packaging system.

But, I see that the module is also on Github https://github.com/pdureau/ui_examples. So? Which repository is the canonical one?

PHPCS:

FILE: project/app/modules/custom/ui_examples/README.md
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FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
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  6 | WARNING | Line exceeds 80 characters; contains 119 characters
 15 | WARNING | Line exceeds 80 characters; contains 82 characters
----------------------------------------------------------------------


FILE: project/app/modules/custom/ui_examples/ui_examples.links.menu.yml
-----------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
-----------------------------------------------------------------------
 5 | ERROR | [x] Expected 1 newline at end of file; 2 found
-----------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
-----------------------------------------------------------------------


FILE: project/app/modules/custom/ui_examples/src/ExamplePluginManagerInterface.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 1 | ERROR | Missing required strict_types declaration
----------------------------------------------------------------------------------


FILE: project/app/modules/custom/ui_examples/ui_examples.module
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FOUND 1 ERROR AFFECTING 1 LINE
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 1 | ERROR | Missing required strict_types declaration
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FILE: project/app/modules/custom/ui_examples/src/Controller/ExamplesLibraryController.php
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FOUND 1 ERROR AND 1 WARNING AFFECTING 2 LINES
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  1 | ERROR   | Missing required strict_types declaration
 13 | WARNING | The class short comment should describe what the class does and not simply repeat the class name
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FILE: project/app/modules/custom/ui_examples/src/ExamplePluginManager.php
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FOUND 1 ERROR AND 1 WARNING AFFECTING 2 LINES
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  1 | ERROR   | [ ] Missing required strict_types declaration
 74 | WARNING | [x] 'TODO: Add validation of the plugin definition here.' should match the format '@todo Fix problem X here.'
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
------------------------------------------------------------------------------------------------------------------------------

PHPStan:

 ------ -------------------------------------------------------------------------------------------------------------------------- 
  Line   modules/custom/ui_examples/src/Controller/ExamplesLibraryController.php                                                   
 ------ -------------------------------------------------------------------------------------------------------------------------- 
  33     Method Drupal\ui_examples\Controller\ExamplesLibraryController::create() has no return typehint specified.                
  56     Method Drupal\ui_examples\Controller\ExamplesLibraryController::single() has parameter $name with no typehint specified.  
  67     Method Drupal\ui_examples\Controller\ExamplesLibraryController::title() has parameter $name with no typehint specified.   
 ------ -------------------------------------------------------------------------------------------------------------------------- 

 ------ --------------------------------------------------------------------------------------------------------------------------- 
  Line   modules/custom/ui_examples/src/ExamplePluginManager.php                                                                    
 ------ --------------------------------------------------------------------------------------------------------------------------- 
  17     Drupal\ui_examples\ExamplePluginManager must override __construct if using YAML plugins.                                   
  17     Plugin definitions cannot be altered.                                                                                      
  72     Method Drupal\ui_examples\ExamplePluginManager::processDefinition() has no return typehint specified.                      
  72     Method Drupal\ui_examples\ExamplePluginManager::processDefinition() has parameter $definition with no typehint specified.  
  72     Method Drupal\ui_examples\ExamplePluginManager::processDefinition() has parameter $plugin_id with no typehint specified.   
  83     Method Drupal\ui_examples\ExamplePluginManager::providerExists() has parameter $provider with no typehint specified.       
 ------ --------------------------------------------------------------------------------------------------------------------------- 

Also some typos and using single quotes instead of double quotes.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes