LibWeb: Implement WritableStream.getWriter()

Author: https://github.com/mattco98
Commit: ae2d67c28b
Pull-request: https://github.com/SerenityOS/serenity/pull/18265
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Streams/WritableStream.h

@@ -48,6 +48,7 @@ public:
 
     bool locked() const;
     WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> close();
+    WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStreamDefaultWriter>> get_writer();
 
     bool backpressure() const { return m_backpressure; }
     void set_backpressure(bool value) { m_backpressure = value; }