Create ReflectionEnumBackedCase class

[devin-ai-integration]

Part of #182

Create src/ReflectionEnumBackedCase.php that extends \ReflectionEnumBackedCase:

1. Extends ReflectionEnumUnitCase
2. Implements getBackingValue(): int|string — read from the EnumCase node's expr property using NodeExpressionResolver

[lisachenko]

closed in #189