chore(internal): export HeadersInit type shim (#241)

Author: stainless-bot

src/_shims/fetch.d.ts

@@ -56,4 +56,5 @@ export type {
   _RequestInfo as RequestInfo,
   _ResponseType as ResponseType,
   _BodyInit as BodyInit,
+  _HeadersInit as HeadersInit,
 };

src/_shims/fetch.node.d.ts

@@ -60,4 +60,5 @@ export type {
   _RequestInfo as RequestInfo,
   _ResponseType as ResponseType,
   _BodyInit as BodyInit,
+  _HeadersInit as HeadersInit,
 };