Make Popup.Reposition() public

When a `Popup` is anchored to its parent, you usually want to reposition when the parent is resized or moved. This was [discussed on StackOverflow](https://stackoverflow.com/questions/1600218/how-can-i-move-a-wpf-popup-when-its-anchor-element-moves) 12 years ago, and the proposed hacks are still valid. E.g.:

```c#
class MyPopup : Popup
{
private Window root;

public MyPopup()
{
Loaded += OnLoaded;
Unloaded += OnUnloaded;
}

private void OnLoaded(object sender, RoutedEventArgs e)
{
root = Window.GetWindow(this);
root.LocationChanged += OnRootLocationChanged;
}

private void OnRootLocationChanged(object sender, EventArgs e)
{
// HACK: Reposition() is internal. This is a way to trigger it!
var offset = this.HorizontalOffset;
this.HorizontalOffset = offset + 0.001;
this.HorizontalOffset = offset;
}

private void OnUnloaded(object sender, RoutedEventArgs e)
{
root.LocationChanged -= OnRootLocationChanged;

Loaded -= OnLoaded;
Unloaded -= OnUnloaded;
}
}
```

Obviously, the [Reposition()](https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Primitives/Popup.cs#L1923) method **should be public**. Please make it so.