fix(ts): auto-complete next/headers (#60817)

### What?

Add support for auto-importing `headers()` and `cookies()`

### Why?

Improve the auto-import experience in IDEs like VScode

### How?

Re-export the `next/headers` type references

Closes NEXT-2146

packages/next/index.d.ts

@@ -9,6 +9,7 @@
 /// <reference path="./dynamic.d.ts" />
 /// <reference path="./error.d.ts" />
 /// <reference path="./head.d.ts" />
+/// <reference path="./headers.d.ts" />
 /// <reference path="./image.d.ts" />
 /// <reference path="./link.d.ts" />
 /// <reference path="./navigation.d.ts" />