Deprecate global app typings

See https://github.com/flarum/core/issues/2857#issuecomment-889841326

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

@@ -1,9 +1,31 @@
+/**
+ * @deprecated Please import `app` from a namespace instead of using it as a global variable.
+ *
+ * @example App in forum JS
+ * ```
+ * import app from 'flarum/forum/app';
+ * ```
+ *
+ * @example App in admin JS
+ * ```
+ * import app from 'flarum/admin/app';
+ * ```
+ */
+declare const app: never;
+
 declare const m: import('mithril').Static;
-declare const app: import('../common/Application').default;
 declare const dayjs: typeof import('dayjs');
 
 // Extend JQuery with our custom functions, defined with $.fn
 interface JQuery {
+  /**
+   * Flarum's tooltip JQuery plugin.
+   *
+   * Do not use this directly. Instead use the `<Tooltip>` component that
+   * is exported from `flarum/common/components/Tooltip`.
+   *
+   * This may be removed in a future version of Flarum.
+   */
   tooltip: import('./tooltips/index').TooltipJQueryFunction;
 }