Add Justified HorizontalAlignment for RichTextBox

### Background and motivation

I noticed that RTF documents support Justified alignment, yet RichTextBoxes do not provide the ability to set that alignment for some reasons.

### API Proposal

```csharp
public enum HorizontalAlignment
{
Left = 0,
Right = 1,
Center = 2,
Justified = 3
}
```

### API Usage

```csharp
RichTextBox richTextBox = new RichTextBox();
richTextBox.SelectionAlignment = HorizontalAlignment.Justified;
```

### Alternative Designs

_No response_

### Risks

None.

### Will this feature affect UI controls?

It should be supported by the designer. It won't have any impact on accessibility. And obviously it doesn't need to be localized.