feat: add StructuredOutputScorer for validating JSON outputs (#23)

## Summary
- Adds a new `StructuredOutputScorer` for evaluating structured JSON
outputs from language models
- Supports both strict and fuzzy matching modes for flexible validation
- Includes comprehensive test coverage with 483 lines of tests

## Changes
- **New scorer implementation**: `src/scorers/structuredOutputScorer.ts`
(362 lines)
  - Validates JSON structure and field values
  - Supports strict equality checking and fuzzy matching
  - Handles nested objects, arrays, and complex data structures
  - Configurable matching modes and validation options
  
- **Comprehensive test suite**:
`src/scorers/structuredOutputScorer.test.ts` (483 lines)
  - Tests strict and fuzzy matching modes
  - Covers edge cases like null values, empty objects, and arrays
  - Tests partial matching and extra field handling
  - Validates custom matcher functionality

- **Integration updates**: 
  - Exported from `src/scorers/index.ts`
  - Added to main exports in `src/index.ts`

## Features
- **Matching modes**:
  - `strict`: Exact equality required (default)
- `fuzzy`: Case-insensitive strings, numeric tolerance, regex patterns,
subset matching
  - Custom function support for specialized validation logic

- **Configuration options**:
  - `requireAll`: Whether all expected fields must be present
  - `allowExtras`: Whether to allow additional fields beyond expected
  - `debug`: Enable detailed logging for troubleshooting

## Type of change
- ✨ New feature (non-breaking change which adds functionality)

## Testing
- [x] Comprehensive unit tests added (100% coverage of new code)
- [x] All existing tests pass
- [x] TypeScript type checking passes
- [x] Linting passes

## Related issues
- N/A

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

.gitignore

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