refactor: prepend a header to each WebTransport chunk

This commit updates the format of the header added in [1], in order to
match the format used for a WebSocket frame ([2]).

Two advantages:

- small payloads only need 1 byte instead of 4
- payloads larger than 2^31 bytes are supported

[1]: https://github.com/socketio/engine.io-parser/commit/6142324fa61204393028f3f58f336d053030ea5f
[2]: https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API/Writing_WebSocket_servers#decoding_payload_length