Add support for WebP image format

For example, you could use the SkiaSharp.Views.Desktop.Common package like this:
```c#
using var skImage = image.ToSKImage();
using var skData = skImage.Encode(SkiaSharp.SKEncodedImageFormat.Webp, 90);
using var file = File.OpenWrite(filename);
skData.SaveTo(file);
```

See https://stackoverflow.com/a/76027832