docs: missing license header

packages/backend/src/@types/twemoji.d.ts

@@ -1,3 +1,8 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 declare module '@twemoji/parser/dist/lib/regex' {
 	const regex: RegExp;
 	export default { default: regex };