[API Proposal]: Public the ScanCode property in KeyEventArgs

### Background and motivation

As https://github.com/unoplatform/uno/pull/14371#discussion_r1407800681 , the UNO for WPF will use reflection to get the ScanCode property in KeyEventArgs.

And the ScanCode property in KeyEventArgs is useful in many businesses code or framework code.

This property is rarely used in WPF today.

The code to get the ScanCode (only ways) : https://github.com/dotnet/wpf/blob/187be47c5554d4745e9b17385889a8086d6e4286/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/HwndSource.cs#L2362

The code to use ScanCode (only ways) : https://github.com/dotnet/wpf/blob/187be47c5554d4745e9b17385889a8086d6e4286/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/TextCompositionManager.cs#L435

### API Proposal

```csharp
public class KeyEventArgs
{
public int ScanCode { get; internal set; }
}
```

Or

```csharp
public class KeyEventArgs
{
public int ScanCode { get; set; }
}
```

### API Usage

```csharp
KeyEventArgs args = ...;
var scanCode = args.ScanCode.
```

### Alternative Designs

_No response_

### Risks

_No response_