✍️ Json serialization: ignore read-only properties

This change was made to ignore properties inherited from `ReactiveObject`, which has annotations that opt out of serialization for public properties. But System.Text.Json doesn't support these annotations (ref dotnet/runtime#29975).

YoutubeDl.Wpf/Utils/FileHelper.cs

@@ -12,6 +12,7 @@ public static class FileHelper
         public static readonly JsonSerializerOptions commonJsonSerializerOptions = new()
         {
             Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
+            IgnoreReadOnlyProperties = true,
             WriteIndented = true,
         };