Add WebRTC polyfill support for Node.js environments

Added optional polyfill parameters to RondevuOptions to support Node.js:
- RTCPeerConnection: Custom peer connection implementation
- RTCSessionDescription: Custom session description implementation
- RTCIceCandidate: Custom ICE candidate implementation

This allows users to plug in wrtc or node-webrtc packages for full
WebRTC support in Node.js environments. Updated documentation with
usage examples and environment compatibility matrix.

Version bumped to 0.7.4

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

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/rondevu-client",
-  "version": "0.7.3",
+  "version": "0.7.4",
   "description": "TypeScript client for Rondevu topic-based peer discovery and signaling server",
   "type": "module",
   "main": "dist/index.js",