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/ScriptPathAttributeGenerator.cs

@@ -17,6 +17,9 @@ namespace Godot.SourceGenerators
             if (context.AreGodotSourceGeneratorsDisabled())
                 return;
 
+            if (context.IsGodotToolsProject())
+                return;
+
             // NOTE: NotNullWhen diagnostics don't work on projects targeting .NET Standard 2.0
             // ReSharper disable once ReplaceWithStringIsNullOrEmpty
             if (!context.TryGetGlobalAnalyzerProperty("GodotProjectDir", out string? godotProjectDir)
@@ -31,7 +34,7 @@ namespace Godot.SourceGenerators
                     tree.GetRoot().DescendantNodes()
                         .OfType<ClassDeclarationSyntax>()
                         // Ignore inner classes
-                        .Where(cds => !(cds.Parent is ClassDeclarationSyntax))
+                        .Where(cds => !cds.IsNested())
                         .SelectGodotScriptClasses(context.Compilation)
                         // Report and skip non-partial classes
                         .Where(x =>