[API Proposal]: BitmapEncoder.Save(string path) override

### Background and motivation

`BitmapEncoder.Save` only takes a `Stream`. This is a proposal to add an override that takes a file path, similar to how there is `XmlDocument.Save(Stream)` and `XmlDocument.Save(string)`.

(The `BitmapDecoder` supports `Stream` and `Uri` sources.)

### API Proposal

```diff
namespace System.Windows.Media.Imaging
{
public abstract partial class BitmapEncoder : DispatcherObject
{
protected BitmapEncoder() { }
public virtual BitmapCodecInfo CodecInfo { get; }
public virtual ReadOnlyCollection ColorContexts { get; set; }
public virtual IList Frames { get; set; }
public virtual BitmapMetadata Metadata { get; set; }
public virtual BitmapPalette Palette { get; set; }
public virtual BitmapSource Preview { get; set; }
public virtual BitmapSource Thumbnail { get; set; }
public static BitmapEncoder Create(System.Guid containerFormat);
public virtual void Save(Stream stream);
+ public void Save(string filename);
}
}
```

### API Usage

```csharp
PngBitmapEncoder png = new();
png.Frames.Add(image);
png.Save("image.png");
```

instead of the currently required

```csharp
PngBitmapEncoder png = new();
png.Frames.Add(image);
using (FileStream stream = File.Create("image.png"))
{
png.Save(stream);
}
```

### Alternative Designs

_No response_

### Risks

Low, no behavior changed, minimum size requirements.