Improve lookup injected setter handler with attribute.

Dodge duck typing.

Illustrates:

```csharp
// Disabling duck-type lookup by attribute explicitly.
[OnPropertyChanged(nameof(Title))]
private ValueTask OnTitleChanged(string value)
{
    // ...
}
```

Idea from Fun Fan F# video conference: https://www.youtube.com/watch?v=nFD7-nQFzZo&list=PLbLAMB4nUGAMqY_qX470gvce8ZFRfvXaM