[Config] Make rector.php extensible instead of fully replaceable

[coisa]

Summary

• Introduced RectorConfigFactory class with configure() and configureWithWorkingDirectory() methods
• Updated rector.php to return configuration closure for consumer extensibility
• Added usage examples in docblocks for require + extend pattern
• Added tests for RectorConfigFactory

Implementation

The solution provides multiple ways for consumers to extend the Rector configuration:

1. Direct usage: return require __DIR__ . '/vendor/fast-forward/dev-tools/rector.php';
2. Factory pattern:

   return static function (RectorConfig $rectorConfig): void {
       RectorConfigFactory::configure($rectorConfig);
       // Add custom configuration
   };

3. Extend with custom rules:

   $configure = require __DIR__ . '/vendor/fast-forward/dev-tools/rector.php';
   return static function (RectorConfig $rectorConfig) use ($configure): void {
       $configure($rectorConfig);
       $rectorConfig->rules([CustomRule::class]);
   };

This allows consumers to build on top of the base configuration without copying the entire file, preserving the ability to receive upstream updates automatically.

Closes #7

[coisa]

Closing this PR because the changes introduced here became inconsistent and hard to review due to issues in the automated generation process.

Instead of trying to fix this incrementally, the work will be redone in a clean, focused follow-up PR to ensure better structure, clarity, and reviewability.