[Core] Unseal Span (#4331) fixes #4083

Xamarin.Forms.Core/Span.cs

@@ -4,7 +4,7 @@ using Xamarin.Forms.Internals;
 namespace Xamarin.Forms
 {
 	[ContentProperty("Text")]
-	public sealed class Span : GestureElement, IFontElement, IStyleElement, ITextElement, ILineHeightElement, IDecorableTextElement
+	public class Span : GestureElement, IFontElement, IStyleElement, ITextElement, ILineHeightElement, IDecorableTextElement
 	{
 		internal readonly MergedStyle _mergedStyle;