Introduce editor.status (or state)

[pjasiun]

We should introduce editor.status observable property, similar to the CKEditor 4: https://docs.ckeditor.com/ckeditor4/latest/api/CKEDITOR_editor.html#property-status

I propose to have the following possible status values:

• initializing - during initialization chain.
• ready - after initializing chain, the editor is ready to work.
• destroyed - after destroy, editor and all plugins are destroyed.

This is important because there is no way to stop promises when the editor is destroyed and some plugins should check the editor status in then.

It can be alternatively called state, both state and status are fine for me. cc @Reinmar