LibWeb: Implement WritableStream.abort()

Author: https://github.com/mattco98
Commit: c421b6113c
Pull-request: https://github.com/SerenityOS/serenity/pull/18265
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Streams/WritableStream.h

@@ -47,6 +47,7 @@ public:
     virtual ~WritableStream() = default;
 
     bool locked() const;
+    WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> abort(JS::Value reason);
     WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> close();
     WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStreamDefaultWriter>> get_writer();