Port frontend-plugin-notifications into an app

[arbrandes]

Description

1. Take the notifications plugin and convert it into a frontend-base app.
2. Instead of adding frontend-base support to tutor-contrib-notifications, just move the whole thing upstream.

TODO

• feat!: migrate to frontend-base frontend-plugin-notifications#107
• Add frontend-app-notifications into tutor-mfe@frontend-base
• feat: add CORE_PLUGINS hook and bundle the notifications tray overhangio/tutor-mfe#292