Fetch implementation with cross-platform support

[nasatome]

What kind of change does this PR introduce?

Feature - Major modernization of the fetch implementation with cross-platform support and security enhancements

What is the current behavior?

• Uses deprecated @supabase/node-fetch (last updated 2023)
• Limited platform support and compatibility issues
• Potential security vulnerabilities in header handling
• Bundle contamination in browser environments
• Complex async initialization patterns

What is the new behavior?

🚀 fetch.ts - Modern Cross-Platform Fetch Implementation

• Replaces deprecated @supabase/node-fetch with node-fetch-native for broader platform support
• Priority system: Custom fetch → Native fetch (Node 18+, Deno, browsers) → Polyfill (Node 14-17)
• Security-hardened against header injection attacks with case-insensitive deduplication
• Bundle protection with ultra-dynamic imports to prevent browser bundle contamination
• Streaming support preserving non-enumerable RequestInit properties
• Worker compatibility for all worker environments (Dedicated, Shared, Service, Edge)

🔒 Security Enhancements

• Prevents authentication bypass via Request object header hiding
• Case-insensitive header normalization for security headers only
• Manual Headers copying to prevent Undici/WHATWG prototype mixing crashes
• Ghost duplicate header prevention (e.g., apikey + ApiKey + APIKEY)

🏗️ SupabaseClient.ts - Seamless Integration

• Sync-compatible wrapper around async fetchWithAuth for backward compatibility
• Lazy initialization pattern for optimal performance
• Custom fetch support via global.fetch option (matches official documentation)
• Maintains all existing APIs while using modernized fetch under the hood

✅ fetch-modernization.test.ts - Comprehensive Testing

• 100% compatibility testing for custom fetch scenarios
• Cross-platform environment detection tests
• Security vulnerability prevention verification
• Backward compatibility assurance for existing integrations
• Header merging safety tests across different runtime implementations

📦 Bundle & Performance Optimizations

• Zero bundle inflation in browser environments
• Minimal webpack warnings (only intentional dynamic import protection)
• Platform detection avoids unnecessary polyfill loading

🌐 Platform Support Matrix

• ✅ Node.js 14-22 (native fetch 18+ or polyfill fallback) [test on node 20+]
• ✅ Deno 1.x (native fetch) [this has yet to be tested]
• ✅ Bun (native fetch) [this has yet to be tested]
• ✅ All browsers (native fetch) [ Chrome and Firefox Tested, not Safari]
• ✅ All worker environments (Dedicated, Shared, Service, Edge) [this has yet to be tested]
• ✅ React Native, Cloudflare Workers (custom fetch support) [this has yet to be tested]

Additional context

• Maintains 100% backward compatibility - no breaking changes to existing APIs
• Tested locally, with my own project and it works, community is requested to test in their own environments (tested using my own build, “@supabase/supabase-js”: “file:/prj/open-source/supabase-js-nasatome”).
• I did it because it was giving me errors with my webpack bundle, and I needed it to work asap, if it doesn't merge, I hope you take into account some of the points I mentioned.