Convert easy entrypoints files to Typescript (#30102)

app/javascript/entrypoints/application.ts

@@ -0,0 +1,15 @@
+import './public-path';
+import main from 'mastodon/main';
+
+import { start } from '../mastodon/common';
+import { loadLocale } from '../mastodon/locales';
+import { loadPolyfills } from '../mastodon/polyfills';
+
+start();
+
+loadPolyfills()
+  .then(loadLocale)
+  .then(main)
+  .catch((e: unknown) => {
+    console.error(e);
+  });