Fix Godot.SourceGenerators for generic classes

Fix invalid C# generated by source generators for generic classes and
add generic classes to the Sample project for testing.

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

@@ -0,0 +1,16 @@
+namespace Godot.SourceGenerators.Sample
+{
+    partial class Generic<T> : Godot.Object
+    {
+    }
+
+    // Generic again but different generic parameters
+    partial class Generic<T, R> : Godot.Object
+    {
+    }
+
+    // Generic again but without generic parameters
+    partial class Generic : Godot.Object
+    {
+    }
+}