feat(react): add position-dependent css classes

[castarco]

• introduce CSS classes to indicate to which "level" belongs each node
• introduce CSS classes to indicate the ordinality of each node among its siblings (wether they are the 1st, 2nd, 3rd... child)

[CrownPab]

Hello, may I be assigned this task?

[castarco]

Hi @CrownPab 😄 , sure, it's yours. 👍🏻 . Please don't hesitate to ask if you have any doubt.

Also, if there's anything you want to discuss that does not fit into the thread of this issue, we have a more open Discussions section (I know, there's not much life there yet 😅 , but I'm on it).