Create ReflectionEnumUnitCase class

[devin-ai-integration]

Part of #182

Create src/ReflectionEnumUnitCase.php that extends \ReflectionEnumUnitCase:

1. Wraps an EnumCase AST node
2. Implements getEnum(): ReflectionEnum — returns the parent enum reflection
3. Implements getValue(): UnitEnum — this requires loading the class (similar to how __initialize() works in other reflection classes)

[lisachenko]

closed in #189