Move extension method classes into separate files

src/Umbraco.Core/Models/ContentEditing/PropertyGroupBasicExtensions.cs

@@ -0,0 +1,8 @@
+namespace Umbraco.Cms.Core.Models.ContentEditing
+{
+    internal static class PropertyGroupBasicExtensions
+    {
+        public static string GetParentAlias(this PropertyGroupBasic propertyGroup)
+            => PropertyGroupExtensions.GetParentAlias(propertyGroup.Alias);
+    }
+}