chore(next): adds export for mergeHeaders utility function (#8609)

Need this utility function exported for this PR: #8490

Author: PatrikKozak

packages/next/src/exports/utilities.ts

@@ -4,3 +4,4 @@ export { createPayloadRequest } from '../utilities/createPayloadRequest.js'
 export { getNextRequestI18n } from '../utilities/getNextRequestI18n.js'
 export { getPayloadHMR, reload } from '../utilities/getPayloadHMR.js'
 export { headersWithCors } from '../utilities/headersWithCors.js'
+export { mergeHeaders } from '../utilities/mergeHeaders.js'