Modernize codebase: OOP hooks, PHP attributes, stricter types, security fix, and expanded tests

Problem/Motivation

The module has several areas that need modernization to align with current Drupal 10.3+ / 11 best practices:

• Hook implementations live as procedural code in .module instead of using the OOP hook pattern.
• The test plugin (DummyType) uses the deprecated @Annotation discovery instead of PHP attributes.
• The formatter outputs plugin labels with #markup, which does not escape user-controlled text, posing a potential XSS risk.
• PHPStan was running at level 1; several loose types (@return mixed, loose equality ==, static:: calls on instance methods) would be caught at higher levels.
• The storageSettingsSummary() displays the raw machine name of the plugin type instead of its human-readable label.
• The widget's supportsConfiguration() does not guard against an empty definitions array, causing a potential error.
• Attribute strings use double quotes instead of single quotes (minor style inconsistency).
• The module requires ^10 but realistically depends on APIs available from 10.3+.

Proposed resolution

1. OOP hook migration

• Add plugin_configuration_field.services.yml with the PluginConfigurationFieldHooks service.
• Move hook_field_widget_info_alter() and hook_field_formatter_info_alter() logic into that class.
• Keep thin procedural wrappers in .module for backward compatibility with Drupal versions without #[Hook] auto-discovery.

2. Annotation to PHP attribute

• Replace the @DummyType annotation class with a DummyType PHP attribute in the test module.
• Update BasicDummyType to use the #[DummyType] attribute.
• Delete the now-unused Annotation/DummyType.php.

3. Stricter types and code quality (PHPStan level 5)

• Raise PHPStan level from 1 to 5.
• Fix loose equality (== to ===) in isEmpty().
• Change getPluginManager() from static to instance method.
• Add proper @return type to the deriver's getEvent().
• Add static return type to create().
• Replace get_class($this) with static::class.
• Normalize attribute strings to single quotes.
• Add type annotations (@var) in the widget for field items.

4. UX: human-readable label in storage settings summary

• storageSettingsSummary() now dispatches the plugin types event to resolve the label instead of showing the machine name.

5. Widget empty-definitions guard

• supportsConfiguration() returns FALSE early when definitions array is empty.

6. Core version requirement bump

• core_version_requirement changed from ^10 to ^10.3 || ^11.

7. API documentation

• Add plugin_configuration_field.api.php documenting the types event hook.

8. Expanded test coverage

• New kernel tests: PluginConfigurationFieldElementTest, PluginConfigurationFieldFormatterTest.
• New functional test: PluginConfigurationFieldWidgetTest.
• New unit test: PluginConfigurationFieldTypesEventTest.
• Additional assertions in existing kernel test: testIsEmpty(), testStorageSettingsSummaryShowsLabel(), testSetValueDefaultsConfiguration().

9. README rewrite

• Complete rewrite with architecture overview, registration guide, usage examples, and submodule documentation.

User interface changes

Storage settings summary now shows the human-readable plugin type label (e.g., "Reference type: Basic dummy type") instead of the machine name.

API changes

• getPluginManager() changed from protected static to protected (instance method). Subclasses calling static::getPluginManager() must update to $this->getPluginManager().
• New plugin_configuration_field.services.yml registers the OOP hooks service.
• New plugin_configuration_field.api.php documents the types event hook.

Data model changes

None. The field schema (two columns: target_plugin_id and target_plugin_configuration) is unchanged.