LibWeb: Implement WritableStreamDefaultWriter.close()

Author: https://github.com/mattco98
Commit: 5f4da20a56
Pull-request: https://github.com/SerenityOS/serenity/pull/18265
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.idl

@@ -9,9 +9,9 @@ interface WritableStreamDefaultWriter {
     readonly attribute Promise<undefined> ready;
 
     Promise<undefined> abort(optional any reason);
+    Promise<undefined> close();
 
     // FIXME:
-    // Promise<undefined> close();
     // undefined releaseLock();
     // Promise<undefined> write(optional any chunk);
 };