Improve type of WritableStreamDefaultController.signal

[MattiasBuelens]

Previously, we defined our own AbortSignal type, which was a small subset of the one defined in TypeScript's DOM types and @types/node. However, this prevents users from using the full AbortSignal API, for example passing { once: true } to signal.addEventListener().

Recently, I came across a neat trick in @types/node to query the existence of a globally defined type:

var AbortSignal: typeof globalThis extends { onmessage: any; AbortSignal: infer T } ? T : /* fallback */;

I'm now using the same trick to get the exact AbortSignal type, while still allowing users to use either lib.dom.d.ts or @types/node in their project. 😁