[Fetch-Based API] Promise

[mingyc]

Context: #70 #71

To maintain the same semantic, browser should probably resolve Promise when the pending request is sent.

But in reality, the Promise may or may not be resolved, or resolved when the page is in bfcache and JS context is frozen. User should not rely on it.

[mingyc]

fetchLater() API returns FetchLaterResult instead of Promise. See spec https://whatpr.org/fetch/1647.html#dom-fetchlaterresult-activated and the discussions in

• whatwg/fetch#1647 (comment)
• whatwg/fetch#1647 (review)