export serializeNodeWithId so rrweb could serialize newly added nodes

index.d.ts

@@ -3,3 +3,8 @@ export * from './src/types';
 
 export function snapshot(n: Document): [serializedNodeWithId | null, idNodeMap];
 export function rebuild(n: serializedNodeWithId): Node | null;
+export function serializeNodeWithId(
+  n: Node,
+  doc: Document,
+  map: idNodeMap,
+): serializedNodeWithId | null;