feat: add include_library option for unnecessary_library_directive rule

[riscait]

Summary

Add include_library option to control whether the library directive is included in generated barrel files.

Background

This addresses the unnecessary_library_directive lint rule that flags library directives as unnecessary in modern Dart. Some projects prefer to omit library directives entirely from their barrel files.

Changes

• ✅ Add include_library boolean field to LibrarySettings (defaults to true)
• ✅ When include_library: false, both library directive and docs are omitted (docs are library-specific documentation)
• ✅ Add comprehensive test coverage for the new functionality
• ✅ Update README with usage examples and documentation

Usage

index_generator:
  libraries:
    - directory_path: lib
      include_library: false  # Omit library directive
    - directory_path: src
      include_library: true   # Include library directive (default)
      name: my_library

Generated Output Examples

With include_library: true (default behavior)

// GENERATED CODE - DO NOT MODIFY BY HAND

/// Library documentation
library my_library;

export 'file1.dart';
export 'file2.dart';

With include_library: false

// GENERATED CODE - DO NOT MODIFY BY HAND

export 'file1.dart';
export 'file2.dart';

Backward Compatibility

• ✅ Fully backward compatible - defaults to true to maintain existing behavior
• ✅ No breaking changes to existing configurations

Testing

• ✅ Unit tests for LibrarySettings default values
• ✅ All existing tests pass