Replace complex Project Settings with a plug-in API

[RedMser]

Many project settings are too complex for their own good, using RegEx or Dictionary for very complex lookups and string manipulation.

Instead it would be good to have a plug-in API that allows the user to implement virtual methods like func _compute_locale(path: String) -> String which could in the end just do a RegEx lookup, but also anything else.

Not sure about the registration mechanism, since it would need to be available as soon as ResourceLoader is ready (during project startup). Maybe refer to plug-in classes by path or by class_name in ProjectSettings?

May depend on godot-rust/gdext#426 OR we implement the interface via GDScript and dynamically instantiate and calls things without Rust typing
Might also be blocked by godotengine/godot#91336 (not sure how early GDScript is ready to be executed)

[RedMser]

One small UX improvement before this is done, we could use typed dictionaries to make the project settings nicer to edit.

Syntax: https://github.com/godotengine/godot/pull/100926/files

[RedMser]

This could also be a great place to allow configuring the automatic load() instantiation of TranslationFluent.

So in its constructor, we query the plug-in API and let the user initialize the resource as they want (e.g. by calling add_function).

Actually, then we wouldn't need _compute_locale either, but instead have something like this:

func _load_ftl(translation: TranslationFluent) -> void:
    translation.set_locale_by_folder("^.+$")
    translation.add_function(...)

[RedMser]

Started to work on https://github.com/RedMser/godot-fluent-translation/tree/plugin-api but it seems like either GDScript is not ready at the time of calling, or I made some mistake that I can't understand...

Also tried with godotengine/godot#91374 and got a crash instead of a null value.