added sourcemapping to nuget.config

change LangVersion from 13.0 to 13

Directory.Build.props

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
-    <LangVersion>13.0</LangVersion>
+    <LangVersion>13</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup>

nuget.config

@@ -1,6 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
-  <packageSources>
-    <add key="AvaloniaCI" value="https://www.myget.org/F/avalonia-ci/api/v2" />
-  </packageSources>
+    <packageSources>
+        <clear />
+        <add key="AvaloniaCI" value="https://www.myget.org/F/avalonia-ci/api/v2" />
+        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    </packageSources>
+    <packageSourceMapping>
+        <!-- key value for <packageSource> should match key values from <packageSources> element -->
+        <packageSource key="nuget.org">
+            <package pattern="*" />
+        </packageSource>
+        <packageSource key="AvaloniaCI">
+            <package pattern="Avalonia*" />
+        </packageSource>
+    </packageSourceMapping>
 </configuration>