AI Translate default configuration is not valid

• Fixed the entity_reference_depth that was wrongly positioned in the default config
• Added missing language_settings default value
• Updated default reference_defaults values. Now it defaults to empty, since it was referencing entity types that might not even exist on a given site, making the configuration valid more of a challenge.

The last change might be debatable...

I also took the opportunity to add extra config validation to the schema.
This is useful for when ai_translate settings get set via another means than the settings form, e.g. via a recipe.