C#: Fix ScriptPathAttribute generator with none or nested namespaces

The following two bugs were fixed:
- For classes without namespace we were still generating `namespace {`
without a namespace identifier, causing a syntax error.
- For classes with nested namespaces we were generating only the innermost
part of the namespace was being generated, e.g.: for `Foo.Bar` we were
generating `namespace Bar {` instead of `namespace Foo.Bar {`.
This wasn't causing any build error, but because of the wrong namespace
Godot wasn't able to find the class associated with the script.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/ExtensionMethods.cs

@@ -82,5 +82,8 @@ namespace Godot.SourceGenerators
 
         public static string FullQualifiedName(this INamedTypeSymbol symbol)
             => symbol.ToDisplayString(NullableFlowState.NotNull, FullyQualifiedFormatOmitGlobal);
+
+        public static string FullQualifiedName(this INamespaceSymbol namespaceSymbol)
+            => namespaceSymbol.ToDisplayString(FullyQualifiedFormatOmitGlobal);
     }
 }