[Set] Add SymfonySetList::COMPOSER_BASED constant

[TomasVotruba]

Exposes the composer-based set as a constant, same as PHPUnitSetList::COMPOSER_BASED in rector-phpunit.

 public const string CONFIGS = __DIR__ . '/../../config/sets/symfony/configs.php';
+
+/**
+ * Rules bound to the exact Symfony package version they are available from
+ */
+public const string COMPOSER_BASED = __DIR__ . '/../../config/sets/symfony/composer-based.php';

Needed by rectorphp/rector-src#8259, where withComposerBased(symfony: true) loads this single set instead of the whole SetGroup::SYMFONY group.