Share Avalonia.Controls with Avalonia.Controls.Documents (#19191)

* Share internals with Avalonia.Constrols.Documents

* Inlines might be null

* Use IAddChild<T>

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

build/ExternalConsumers.props

@@ -30,5 +30,6 @@
     <InternalsVisibleTo Include="UIAutomationProvider, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9" />
     <InternalsVisibleTo Include="PresentationCore, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9" />
     <InternalsVisibleTo Include="Avalonia.ForTestingOnly, PublicKey=$(AvaloniaPublicKey)" />
+	<InternalsVisibleTo Include="Avalonia.Controls.Documents, PublicKey=$(AvaloniaPublicKey)" />
   </ItemGroup>
 </Project>

src/Avalonia.Controls/Documents/Span.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Controls.Documents
     /// <summary>
     /// Span element used for grouping other Inline elements.
     /// </summary>
-    public class Span : Inline
+    public class Span : Inline, IAddChild<Inline>, IAddChild<Control>, IAddChild<string>
     {
         /// <summary>
         /// Defines the <see cref="Inlines"/> property.
@@ -96,5 +96,20 @@ namespace Avalonia.Controls.Documents
             void OnInlinesInvalidated(object? sender, EventArgs e)
                 => InlineHost?.Invalidate();
         }
+
+        void IAddChild<Inline>.AddChild(Inline inline)
+        {
+            Inlines?.Add(inline);
+        }
+
+        void IAddChild<Control>.AddChild(Control child)
+        {
+            Inlines?.Add(new InlineUIContainer(child));
+        }
+
+        void IAddChild<string>.AddChild(string text)
+        {
+            Inlines?.Add(new Run(text));
+        }
     }
 }