C#/netcore: Add base desktop game export implementation

This base implementation is still very barebones but it defines the path
for how exporting will work (at least when embedding the .NET runtime).

Many manual steps are still needed, which should be automatized in the
future. For example, in addition to the API assemblies, now you also
need to copy the GodotPlugins assembly to each game project.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/ScriptBoilerplate.cs

@@ -1,6 +1,8 @@
+using System;
+
 namespace Godot.SourceGenerators.Sample
 {
-    public partial class ScriptBoilerplate : Godot.Node
+    public partial class ScriptBoilerplate : Node
     {
         private NodePath _nodePath;
         private int _velocity;
@@ -17,5 +19,18 @@ namespace Godot.SourceGenerators.Sample
             _ = name;
             return 1;
         }
+
+        public void IgnoreThisMethodWithByRefParams(ref int a)
+        {
+            _ = a;
+        }
+    }
+
+    partial struct OuterClass
+    {
+        public partial class NesterClass : RefCounted
+        {
+            public override object _Get(StringName property) => null;
+        }
     }
 }