[DX] Find a more robust way to handle #indentation in CSS

[klonos]

This is a follow-up to #5999 which (hopefully) with the help of the changes that went in with #6028 will help us handle indentation in a better way at some point in the future (and this is a placeholder issue to do just that). As I said in #6028:

Since the current solution was based on CSS classes, it is limited to only handling indentation depths from 0 to 5, which is not as robust. I'm hoping that the addition of this new data-* attribute, which holds the indentation depth as its value, can be used by contrib and custom code to provide a better solution for indentation, either based on custom JS, or in core in the future (when there's better browser support for CSS calculations that include data-* attributes).

[klonos]

to-do: find the time to outline the findings of the research around CSS, calculated values, and data attributes (long story short is that it is not currently possible, but could be in the future)