Add Webflow strategy for posting to CMS collections

[Copilot]

This PR implements a comprehensive Webflow strategy for the crosspost project, enabling users to post content to Webflow CMS collections via the Webflow API v2.

Features

Core Functionality

• CMS Integration: Posts content to Webflow CMS collections using the official API v2
• OAuth Authentication: Supports OAuth access tokens with proper scope validation
• Image Support: Uploads images as Webflow assets and references them in collection items
• Error Handling: Comprehensive error handling with descriptive messages for API failures
• AbortSignal Support: Proper cancellation support following crosspost patterns

API Integration Details

• Uses Webflow CMS API v2 endpoints (/collections/{id}/items and /sites/{id}/assets)
• Implements proper authentication headers and request formatting
• Handles rate limiting scenarios and provides guidance for production use
• Supports custom API base URLs for testing and future API versions

Strategy Implementation

import { WebflowStrategy } from "@humanwhocodes/crosspost";

const strategy = new WebflowStrategy({
    accessToken: "your-oauth-token",
    siteId: "your-site-id", 
    collectionId: "your-collection-id"
});

// Post with images
await strategy.post("Hello Webflow!", {
    images: [{ data: imageData, alt: "Description" }]
});

Documentation

The implementation includes comprehensive documentation covering:

• API References: Links to official Webflow documentation for collection items, OAuth, and app registration
• Authentication Requirements: OAuth scopes, application registration process
• Rate Limits: Best practices for production usage and retry strategies
• Configuration: Required parameters and collection structure requirements
• Limitations: Current constraints and future enhancement opportunities

Testing

Added 24 comprehensive test cases covering:

• Constructor validation and error handling
• Message posting with various scenarios
• Image upload functionality and error cases
• API error responses and network failures
• AbortSignal cancellation support
• Request body validation and slug generation
• Response URL generation

All existing tests continue to pass (314 total tests passing).

Implementation Notes

• Follows established crosspost strategy patterns for consistency
• Uses proper TypeScript/JSDoc type definitions
• Implements automatic slug generation from message content
• Handles long messages by truncating names while preserving full content
• Maps images to common Webflow field patterns (main-image and images arrays)

The strategy is production-ready and integrates seamlessly with the existing crosspost ecosystem.

Fixes #117.

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