Add license headers across the project

ts/OS.ts

@@ -1,3 +1,6 @@
+// Copyright 2018-2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
 import is from '@sindresorhus/is';
 import os from 'os';
 import semver from 'semver';