refactor(v2): Convert sitemap plugin to TypeScript (#1894)

* Convert sitemap plugin to TypeScript

Test - enabled the sitemap plugin in the v2 website and verified that
the sitemap is created after running `docusaurus build`.

* Addressing review comments

packages/docusaurus-plugin-sitemap/src/index.ts

@@ -0,0 +1,46 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import fs from 'fs';
+import path from 'path';
+import {PluginOptions} from './types';
+import createSitemap from './createSitemap';
+import {LoadContext, Props} from '@docusaurus/types';
+
+const DEFAULT_OPTIONS: PluginOptions = {
+  cacheTime: 600 * 1000, // 600 sec - cache purge period
+  changefreq: 'weekly',
+  priority: 0.5,
+};
+
+export default function pluginSitemap(
+  _context: LoadContext,
+  opts: Partial<PluginOptions>,
+) {
+  const options = {...DEFAULT_OPTIONS, ...opts};
+
+  return {
+    name: 'docusaurus-plugin-sitemap',
+
+    async postBuild({siteConfig, routesPaths, outDir}: Props) {
+      // Generate sitemap
+      const generatedSitemap = createSitemap(
+        siteConfig,
+        routesPaths,
+        options,
+      ).toString();
+
+      // Write sitemap file
+      const sitemapPath = path.join(outDir, 'sitemap.xml');
+      fs.writeFile(sitemapPath, generatedSitemap, err => {
+        if (err) {
+          throw new Error(`Sitemap error: ${err}`);
+        }
+      });
+    },
+  };
+}