add fetch api

[plantain-00]

Fixes microsoft/TypeScript#8966.
The definitions are from https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/whatwg-fetch/whatwg-fetch.d.ts

[mhegazy]

👍

[mhegazy]

@zhengbli would this be autogenerated if we took a drop of the Edge definition files today?

[zhengbli]

@mhegazy Yes. The latest definite file in the redstone build has a GlobalFetch interface, and window implements it.

[zhengbli]

Request should also have a clone method that returns Request:
https://developer.mozilla.org/en-US/docs/Web/API/Request/clone

[mhegazy]

then we should just take a new drop, instead of hand authoring these definitions..

[zhengbli]

As mentioned above, the new spec files that we haven't used yet contains the definition of fetch APIs, so it might be more consistent if we wait to take the drop. Although we should speed up the process to something like a monthly update. I'll talk to the Edge team to see if there are anything blocking that from happening.

Closing this issue for now.