chore: 著作権とライセンスについての情報を各ファイルに追加する (#11348)

* chore: Add the SPDX information to each file

Add copyright and licensing information as defined in version 3.0 of
the REUSE Specification.

* tweak format

---------

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>

packages/backend/src/misc/is-not-null.ts

@@ -1,3 +1,8 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and other misskey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 // we are using {} as "any non-nullish value" as expected
 // eslint-disable-next-line @typescript-eslint/ban-types
 export function isNotNull<T extends {}>(input: T | undefined | null): input is T {