newValue argument as Ref for *Changing() generated methods

### Overview

Hi, I'm migrating some old C# ViewModel code to use the [ObservableProperty] attribute on all the properties.

I'd like to convert the following code:

```csharp
private string _windowTitle;
public string WindowTitle {
get {
return _windowTitle;
}
set {
_windowTitle = $ "Window title prefix: {value}";
OnPropertyChanged(nameof(WindowTitle));
}
}
```

...but I couldn't find any "simple" or "standard" method to accomplish the same behavior using the MVVM Toolkit.

I discovered that recently the methods *Changing(string? newValue) or *Changing(string? oldValue, string newValue) got added, but the newValue isn't passed as `ref string`, so I can't intercept and change the value.

Could this prototype be implemented for those methods? Thanks!

### API breakdown

```csharp
partial void OnWindowTitleChanging(ref string value);
partial void OnWindowTitleChanging(string? oldValue, ref string newValue);
```

### Usage example

```csharp
partial void OnWindowTitleChanging(string? oldValue, ref string newValue)
{
newValue = "Window title prefix: " + newValue;
}
```

### Breaking change?

I'm not sure

### Alternatives

-

### Additional context

_No response_

### Help us help you

No, just wanted to propose this