feature: Add RegistrationNamespace.Avalonia (#2699)

Author: worldbeater

src/ReactiveUI.Tests/API/ApiApprovalTests.ReactiveUI.net472.approved.txt

@@ -655,6 +655,7 @@ namespace ReactiveUI
         Uno = 3,
         Blazor = 4,
         Drawing = 5,
+        Avalonia = 6,
     }
     public class Registrations
     {

src/ReactiveUI.Tests/API/ApiApprovalTests.ReactiveUI.net5.0.approved.txt

@@ -650,6 +650,7 @@ namespace ReactiveUI
         Uno = 3,
         Blazor = 4,
         Drawing = 5,
+        Avalonia = 6,
     }
     public class Registrations
     {

src/ReactiveUI.Tests/API/ApiApprovalTests.ReactiveUI.netcoreapp3.1.approved.txt

@@ -648,6 +648,7 @@ namespace ReactiveUI
         Uno = 3,
         Blazor = 4,
         Drawing = 5,
+        Avalonia = 6,
     }
     public class Registrations
     {

src/ReactiveUI/RegistrationNamespace.cs

@@ -38,6 +38,11 @@ public enum RegistrationNamespace
         /// <summary>
         /// Drawing.
         /// </summary>
-        Drawing
+        Drawing,
+
+        /// <summary>
+        /// Avalonia.
+        /// </summary>
+        Avalonia
     }
 }