Convert the loading window to TypeScript

ts/windows/loading/preload.ts

@@ -0,0 +1,19 @@
+// Copyright 2020-2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { ipcRenderer } from 'electron';
+import * as url from 'url';
+import * as i18n from '../../../js/modules/i18n';
+import { strictAssert } from '../../util/assert';
+
+const config = url.parse(window.location.toString(), true).query;
+
+const { locale } = config;
+strictAssert(
+  typeof locale === 'string',
+  'Expected to be configured with a string locale'
+);
+
+const localeMessages = ipcRenderer.sendSync('locale-data');
+
+window.i18n = i18n.setup(locale, localeMessages);