Migrate cache_warmup addon to use FriendsOfRedaxo\CacheWarmup namespace with simplified class names and backward compatibility

[Copilot]

This PR migrates the cache_warmup addon to use the proper namespace FriendsOfRedaxo\CacheWarmup and converts all class names to follow PSR-1 naming conventions with a simplified structure.

Changes Made

Namespace Migration & Simplification:

• Added namespace FriendsOfRedaxo\CacheWarmup; declaration to all classes in the lib/ directory
• Simplified class names by removing the CacheWarmup prefix:
  • cache_warmup_generator → Generator
  • cache_warmup_generator_images → GeneratorImages
  • cache_warmup_generator_pages → GeneratorPages
  • cache_warmup_selector → Selector
  • cache_warmup_writer → Writer

Backward Compatibility:

• Created deprecated wrapper classes in lib/deprecated/ that extend the new namespaced classes
• All old class names remain functional until the next major release
• Added proper @deprecated annotations with migration guidance

Updated References:

• Updated use statements in page files (pages/generator.php, pages/warmup.php)
• Class inheritance relationships updated to use new simplified names
• Class instantiation and static method calls updated in page files

Migration Documentation:

• Added comprehensive migration guide to README.md
• Included code examples showing old vs new usage patterns
• Documented all class name changes and namespace structure

Implementation Notes

• Method names were already correctly following camelCase convention
• External REDAXO framework classes (rex_*) remain unchanged
• String identifiers (CSRF token names, CSS classes, translation keys) preserved
• All PHP files pass syntax validation
• Deprecated classes properly include parent classes for immediate usability
• Maintains full backward compatibility through proper namespace usage and wrapper classes

Fixes #120.

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