Replace jQuery code with vanilla JS for better performance (#35)

* Bundle `pusher-js`

* Update `pusher/pusher-php-server`

* StyleCi fix

* Revert "Bundle `pusher-js`"

This reverts commit 55bb1ff0988973e436317c00ca2b83ad4c39d6da.

* Remove no longer used code

* Replace `$.getScript` for better performance

* Revert "Update `pusher/pusher-php-server`"

This reverts commit 7d1e2336

* Use `import()`

* Add `once`

* Update js/tsconfig.json

Co-authored-by: Alexander Skvortsov <38059171+askvortsov1@users.noreply.github.com>

* Async function

Co-authored-by: Alexander Skvortsov <38059171+askvortsov1@users.noreply.github.com>

extensions/pusher/js/tsconfig.json

@@ -7,6 +7,7 @@
     "baseUrl": ".",
     "paths": {
       "flarum/*": ["../vendor/flarum/core/js/dist-typings/*"]
-    }
+    },
+    "module": "es2020"
   }
 }