Add new eslint plugin to check for valid i18n keys

eslint-local-rules.js

@@ -0,0 +1,7 @@
+// Copyright 2022 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+/* eslint-disable global-require */
+
+module.exports = {
+  'valid-i18n-keys': require('./.eslint/rules/valid-i18n-keys'),
+};