Vendor whatwg-fetch@3.6.20 (#56370)

Summary:
Pull Request resolved: #56370

Vendor `whatwg-fetch` v3.6.20 directly into `Libraries/Network/`, replacing the npm dependency with a local copy.

`whatwg-fetch` has not had a release since December 2023 and appears to be [unmaintained](JakeChampion/fetch#1482), by download numbers `react-native` is the dominant consumer (all this is unsurprising, since `fetch` is natively implemented in every browser).

It has several shortcomings we (with the OSS community) could improve on - one important one being that `whatwg-fetch`'s `fetch()` does not resolve immediately on receipt of headers, only on completion.

This takes the single source file from the `whatwg-fetch` npm package as `Libraries/Network/whatwg-fetch.js`, and its accompanying types, marking as originally vendored from whatwg-fetch v3.6.20 (Copyright (c) 2014-2023 GitHub, Inc., MIT license).

Changelog:
[Internal]

Reviewed By: huntie

Differential Revision: D99854694

fbshipit-source-id: 6e742c47c43d69012841671608cc8b359d20ea91

Author: robhogan

packages/react-native/Libraries/Network/fetch.js

@@ -12,7 +12,7 @@
 
 // side-effectful require() to put fetch,
 // Headers, Request, Response in global scope
-require('whatwg-fetch');
+require('./whatwg-fetch');
 
 export const fetch = global.fetch;
 export const Headers = global.Headers;