Add better structured variable support to DialogueTree extension

[blurymind]

Description

It would be nice if DialogueTree could accepts structured (object) variables, even though I am not entirely sure how it would work.
Just raising this here to test the waters before attempting to make a PR i guess

Solution suggested

On the gdevelop side - I am not sure if we can tell gdevelop to pass a structured variable to an extension. Does the api allow for the selection of this type?

bondagejs technically doesnt limit you to what can be saved/loaded
https://github.com/hylyh/bondage.js/blob/master/src/default-variable-storage.js

the yarn language however I believe uses flat structure for variables. I need to do some digging to see if bondagejs also allows deserializing nested structures
https://v1.yarnspinner.dev/docs/writing/expressions-and-variables/

{
 myVariable: {
   childVariable: 123
  }
}

becomes 
<<if $myValiable.childVariable >>

if it doesnt, one idea is to simply let the dialogue tree extension do that part, where it actually still uses a flat structure, but uses dot or something else to serialize/deserialize nested keys?

Anyways I would be interested if this is something that would be desired by the community.
I personally would like to improve it in order to make it easier to build an entire visual novel type game where all game data is persisted by yarn itself (loading/saving from one place)

I think while we have this awesome extension, it would be good to really stress it by trying to build an entire visual game engine with it in gdevelop. I am finding it difficult when trying to do this with a flat structured data

[blurymind]

I am thinking of adding something like this in dialoguetools

https://codepen.io/blurymind/pen/wvZzggG?editors=1112

It will enable storing actor stats or inventory in a structured way, that can then be used for implementing UI in gdevelop.
So if you do this in yarn:

// init
<<set $c.tom.outfit = work_suit>>
<<set $c.maya.outfit = winter_coat>>
<<set $c.tom.mood = happy>>
<<set $c.maya.mood = sad>>

to build a HUD in Gdevelop, you will be able to dig into "c" and loop over all existing characters ([tom, maya]), then you can loop over each stat ([outfit, mood])

Keeping that stuff in yarn state greatly simplifies persisting it and kills the possibility of copy pasta errors

[blurymind]

experimenting with this now in a javascript event - i was able to extend the dialogtools functionality without even running gdevelop in dev mode, which is excellent :D

It does work very nicely, but i am hitting one pesky limitation: Gdevelop's ui extension framework doesnt allow custom expressions to return an array
@Bouh @4ian is that something that is there but hidden? I wonder if there is a way around it?

is this advances nested variables functionality in yarn state something that would be beneficial to pr to gdevelop?
I just wonder if its worth spending the time to create a demo and make a pr to dialoguetools to add these extra utility methods/expressions I am making..