New entity form when Gutenberg is enabled

Problem/Motivation

Up until now Gutenberg was only for nodes. So loading the editor was done with the hook_node_form_alter. One of the main issues with this approach is that if another module or theme also altered the node edit form, the editor could break.
Moving to an entity agnostic approach, we should also change the approach on how we load the editor on an entity edit form.

Proposed resolution

A solution is to do a bit like Layout Builder does, create a new edit form for Gutenberg but additionally override the default edit form with Gutenberg edit form. Probably needs some extra work for the node entity but should work for the majority of entities.
This probably needs some kind of PoC in order to see if there's any gotchas.
So, first implement a simple form and override entity's edit form.
Then, if all good, we'll implement the editor loading.

User interface changes

Although it's a new edit form, for the user the editor remains the same.