Add Facebook strategy for posting to Facebook via Graph API

[Copilot]

This PR implements a comprehensive Facebook strategy for the crosspost project, enabling users to post messages and images to Facebook alongside other supported social media platforms.

Features Implemented

✅ Text Posting: Posts messages to Facebook feed via Graph API v18.0
✅ Image Support: Uploads and posts images with captions using the photos endpoint
✅ Error Handling: Comprehensive error handling for API failures, rate limits, and authentication issues
✅ Abort Support: Cancellation support via AbortSignal for long-running operations
✅ URL Generation: Extracts post URLs from API responses for linking back to posts
✅ Input Validation: Follows existing patterns for validating images and options
✅ TypeScript Support: Full type definitions and seamless compilation

Usage Example

import { FacebookStrategy, Client } from '@humanwhocodes/crosspost';

// Initialize the Facebook strategy
const facebook = new FacebookStrategy({
  accessToken: 'your-facebook-access-token'
});

// Use with crosspost client
const client = new Client([facebook]);
await client.post('Hello Facebook! 🌍');

// Post with image
await client.post('Check out this image!', {
  images: [{ data: imageBuffer, alt: 'Description' }]
});

Authentication Requirements

To use this strategy, you need:

1. A Facebook App registered at https://developers.facebook.com/apps/
2. An access token with appropriate permissions:
   • pages_manage_posts - For posting to Facebook Pages
   • publish_to_groups - For posting to Facebook Groups
   • User access token with appropriate scope for personal posts

API Integration Details

• Endpoint: https://graph.facebook.com/v18.0/me/feed for text posts
• Image Upload: https://graph.facebook.com/v18.0/me/photos for posts with images
• Character Limit: 63,206 characters (very generous compared to other platforms)
• Rate Limits: 200 calls per hour per user (standard tier)

Testing

Added comprehensive test suite with 21 test cases covering:

• Constructor validation
• Text posting success and failure scenarios
• Image upload functionality and error handling
• Input validation for images and options
• Abort signal support
• URL extraction from various response formats
• Message length calculation

All existing tests continue to pass, ensuring no regressions.

Documentation

The strategy includes detailed JSDoc comments explaining:

• Authentication setup and requirements
• Required permissions for different use cases
• Rate limiting considerations
• Best practices for automated posting
• Links to Facebook Developer documentation

Fixes #115.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.