Fix loading of plugins

[jtpio]

Fixes #6399

• Move the list of plugins included in the notebook application to app/package.json
• Pass the list of plugins available on all the pages (tree, notebooks, edit) so the app is aware of other plugins loaded on other pages
• Add logic to explicitly load the notebook built-in plugins with settingRegistry.load(plugin) so they are properly displayed in the settings editor:

Before

After

The settings editor now includes the "Debugger" entry while the debugger extension is only loaded on the notebook page.

[github-actions]

👈 Launch a Binder on branch jtpio/notebook/plugins