Should `StableSocket` just extend `EventTarget`

It's a little un-idiomatic to use delegate objects as a "callback bag" in the Web Platform.

This could potentially, instead, use EventTarget, and emit events for each of the callbacks we have in the `delegate` object. This gives us a lot more flexibility with handling these lifecycle steps.

先閱讀 StableSocket 的實作，以及 issue 中描述的 delegate callbacks。判斷擴充 EventTarget 是否能在保留現有生命週期行為的同時取代 callback-bag 設計；當該設計決策及其所需範圍明確後，issue 即完成。

由索引模型根據 Issue 內容生成。