Gutenberg enabled hook

Problem/Motivation

The _gutenberg_is_gutenberg_enabled() function is used to check if Gutenberg should be enabled for an entity. The problem is that there is no way for other modules to decide on this.

Proposed resolution

Create a new hook, called hook_gutenberg_enabled() that is triggered by the function, and if there is any module that decides to not have the Gutenberg editor enabled, then the editor is disabled. The default would still be to check the Gutenberg settings from the node type configuration.

API changes

A new hook: hook_gutenberg_enabled(), returning a boolean value.