LibWeb: Implement WritableStream.close()

Author: https://github.com/mattco98
Commit: 5faa0014f2
Pull-request: https://github.com/SerenityOS/serenity/pull/18263
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/Streams/WritableStream.idl

@@ -7,6 +7,7 @@ interface WritableStream {
 
     // FIXME:
     // Promise<undefined> abort(optional any reason);
-    // Promise<undefined> close();
     // WritableStreamDefaultWriter getWriter();
+
+    Promise<undefined> close();
 };