avalonia v12 的`MultiBinding`变为封闭类

```cs
System.TypeLoadException: Could not load type 'Lang.Avalonia.MarkupExtensions.MultiBindingExtensionBase' from assembly 'Lang.Avalonia, Version=11.3.12.1, Culture=neutral, PublicKeyToken=null' because the parent type is sealed.
```

在v12中MultiBinding被改为sealed class，因此无法继承了。

```cs
class MultiBindingExtensionBase : MultiBinding
```