Allow overriding the provider's static instance

[amywarble]

This allows package consumers to override some icons by setting the data provider.

The provider instance cannot be null. Attempting to set the provider instance to null will throw an ArgumentNullException.

It can be used as:

public class CustomIconProvider : MaterialIconDataProvider
{
    public override string ProvideData(MaterialIconKind kind)
    {
        return kind switch
        {
            MaterialIconKind.TrophyVariant => "some SVG code",
            _ => base.ProvideData(kind)
        };
    }
}

internal static class Program
{
    public static int Main(string[] args)
    {
        MaterialIconDataProvider.Instance = new CustomIconProvider(); // use custom icons

        return BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
    }
}