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/Common.cs

@@ -28,5 +28,37 @@ namespace Godot.SourceGenerators
                 cds.GetLocation(),
                 cds.SyntaxTree.FilePath));
         }
+
+        public static void ReportNonPartialGodotScriptOuterClass(
+            GeneratorExecutionContext context,
+            TypeDeclarationSyntax outerTypeDeclSyntax
+        )
+        {
+            var outerSymbol = context.Compilation
+                .GetSemanticModel(outerTypeDeclSyntax.SyntaxTree)
+                .GetDeclaredSymbol(outerTypeDeclSyntax);
+
+            string fullQualifiedName = outerSymbol is INamedTypeSymbol namedTypeSymbol ?
+                namedTypeSymbol.FullQualifiedName() :
+                "type not found";
+
+            string message =
+                $"Missing partial modifier on declaration of type '{fullQualifiedName}', " +
+                $"which contains one or more subclasses of '{GodotClasses.Object}'";
+
+            string description = $"{message}. Subclasses of '{GodotClasses.Object}' and their " +
+                                 "containing types must be declared with the partial modifier.";
+
+            context.ReportDiagnostic(Diagnostic.Create(
+                new DiagnosticDescriptor(id: "GODOT-G0002",
+                    title: message,
+                    messageFormat: message,
+                    category: "Usage",
+                    DiagnosticSeverity.Error,
+                    isEnabledByDefault: true,
+                    description),
+                outerTypeDeclSyntax.GetLocation(),
+                outerTypeDeclSyntax.SyntaxTree.FilePath));
+        }
     }
 }