refactor(plugin-google-gtag, plugin-google-analytics): migrate packages to TS (#5561)

* migration

* Move to devDeps

* Use type assertion

packages/docusaurus-plugin-google-analytics/src/analytics.ts

@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import ExecutionEnvironment from '@docusaurus/ExecutionEnvironment';
+
+export default (function () {
+  if (!ExecutionEnvironment.canUseDOM) {
+    return null;
+  }
+
+  return {
+    onRouteUpdate({location}: {location: Location}) {
+      // Set page so that subsequent hits on this page are attributed
+      // to this page. This is recommended for Single-page Applications.
+      window.ga('set', 'page', location.pathname);
+      // Always refer to the variable on window in-case it gets
+      // overridden elsewhere.
+      window.ga('send', 'pageview');
+    },
+  };
+})();