Create ReflectionEnum class

[devin-ai-integration]

Part of #182

Create src/ReflectionEnum.php that extends \ReflectionEnum (which itself extends \ReflectionClass):

1. Accept a class name or Enum_ AST node (similar to how ReflectionClass works)
2. Implement these methods specific to \ReflectionEnum:
   • getBackingType(): ?ReflectionNamedType — read from $enumNode->scalarType
   • getCase(string $name): ReflectionEnumUnitCase|ReflectionEnumBackedCase — find the EnumCase node by name
   • getCases(): array — return all enum cases as ReflectionEnumUnitCase/ReflectionEnumBackedCase instances
   • hasCase(string $name): bool
   • isBacked(): bool — check if $enumNode->scalarType is set
3. Use the same InitializationTrait pattern as ReflectionClass
4. Reference existing enum handling in src/ReflectionClass.php lines 100-112 and src/ReflectionMethod.php lines 404-412