test(backend): enable typecheck by workflow (#13526)

packages/backend/test/global.d.ts

@@ -0,0 +1,7 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type FIXME = any;