Convert external integration to be plugin-based.

I've attempted to do the bare minimum required to enable this module's integrations to be plugin based.

I've included a README.md so that future developers can start developing plugins quickly.

I've created two new plugins:
- Toolbar
- Page top

I've created a post_update.php implementation to make the necessary configuration schema changes.

Honestly, I'm not sure this is the right approach as each new integration seems to require a new entrypoint.
page_top needs hook_page_top
toolbar needs hook_toolbar
The new navigation module is entirely different using blocks and experience builder.

My recommendation is to just move each integration into a sub module instead of as a plugin.
See toolbar as submodule 🌱 Move toolbar integration into submodule. Active
See navigation as sub module ✨ Support for core navigation experimental module Needs work

Additionally, we should add a config option to disable page_top so that disabling does not depend on the toolbar integration.

Yes, I like the idea to move it to sub modules.

trackleft2 → closed merge request !80