fix(node-fetch): support HeaderInit

In previous change to fix the HeadersInit type, I renamed HeaderInit to HeadersInit.

Ref #34382

types/node-fetch/index.d.ts

@@ -172,6 +172,8 @@ export interface ResponseInit {
 }
 
 export type HeadersInit = Headers | string[][] | { [key: string]: string };
+// HeaderInit is exported to support backwards compatibility. See PR #34382
+export type HeaderInit = HeadersInit;
 export type BodyInit = ArrayBuffer | ArrayBufferView | NodeJS.ReadableStream | string | URLSearchParams;
 export type RequestInfo = string | Request;