LibCore: Implement PosixSocketHelper::pending_bytes() on Windows

Author: https://github.com/ayeteadoe
Commit: ba15348d56
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5435
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/R-Goc

Libraries/LibCore/SocketWindows.cpp

@@ -112,7 +112,13 @@ ErrorOr<void> PosixSocketHelper::set_receive_timeout(AK::Duration timeout)
 
 ErrorOr<size_t> PosixSocketHelper::pending_bytes() const
 {
-    VERIFY(0 && "Core::PosixSocketHelper::pending_bytes is not implemented");
+    if (!is_open()) {
+        return Error::from_windows_error(WSAENOTCONN);
+    }
+
+    int value;
+    TRY(System::ioctl(m_fd, FIONREAD, &value));
+    return static_cast<size_t>(value);
 }
 
 void PosixSocketHelper::setup_notifier()