GDScript: Add _ prefix on class name in type compatibility check

This makes sure that the classes internally represented with an
underscore (_) prefix, such as singletons, are still properly checked
for inheritance in the ClassDB.

modules/gdscript/gdscript_parser.cpp

@@ -6109,12 +6109,18 @@ bool GDScriptParser::_is_type_compatible(const DataType &p_container, const Data
 			break;
 	}
 
+	// Some classes are prefixed with `_` internally
+	if (!ClassDB::class_exists(expr_native)) {
+		expr_native = "_" + expr_native;
+	}
+
 	switch (p_container.kind) {
 		case DataType::NATIVE: {
 			if (p_container.is_meta_type) {
 				return ClassDB::is_parent_class(expr_native, GDScriptNativeClass::get_class_static());
 			} else {
-				return ClassDB::is_parent_class(expr_native, p_container.native_type);
+				StringName container_native = ClassDB::class_exists(p_container.native_type) ? p_container.native_type : StringName("_" + p_container.native_type);
+				return ClassDB::is_parent_class(expr_native, container_native);
 			}
 		} break;
 		case DataType::SCRIPT: