Add JS IDE helper (#13012)

* add IDE helper for Webpack

* fix ESLint error in IDE helper

* fix IDE helper code style

ide-helper.js

+/* global path */
+/*
+Preferences | Languages & Frameworks | JavaScript | Webpack | webpack configuration file
+jetbrains://WebStorm/settings?name=Languages+%26+Frameworks--JavaScript--Webpack
+*/
+module.exports = {
+  resolve: {
+    alias: {
+      'mastodon': path.resolve(__dirname, 'app/javascript/mastodon'),
+    },
+  },
+};