Update core to use new Webpack config, flarum-tsconfig, and build action (#2856)

* Update core to use new Webpack and TSConfig

* Bump webpack dep

* Update JS build action

* Copy custom `.d.ts` files to `dist-typings` folder on typings compile

* Run workflow against action v2

This allows us to have a moving tag, like first party actions have.

* Remove bundle analyzer from core

* Use webpack config 0.1.0-beta.16.2

* Add note about checking flarum-tsconfig

js/@types/global/index.d.ts

@@ -0,0 +1,39 @@
+// Mithril
+import Mithril from 'mithril';
+
+// Other third-party libs
+import * as _dayjs from 'dayjs';
+import * as _$ from 'jquery';
+
+// Globals from flarum/core
+import Application from '../../src/common/Application';
+
+import type { TooltipJQueryFunction } from '../tooltips';
+
+/**
+ * flarum/core exposes several extensions globally:
+ *
+ * - jQuery for convenient DOM manipulation
+ * - Mithril for VDOM and components
+ * - dayjs for date/time operations
+ *
+ * Since these are already part of the global namespace, extensions won't need
+ * to (and should not) bundle these themselves.
+ */
+declare global {
+  // $ is already defined by `@types/jquery`
+  const m: Mithril.Static;
+  const dayjs: typeof _dayjs;
+
+  // Extend JQuery with our custom functions, defined with $.fn
+  interface JQuery {
+    tooltip: TooltipJQueryFunction;
+  }
+}
+
+/**
+ * All global variables owned by flarum/core.
+ */
+declare global {
+  const app: Application;
+}