Add a global setting to switch between Foundation & Bootstrap (5) support

Note to myself: A better architectural approach might be to use submodules to split things here. Otherwise the code might become even more spaghetti at all levels and harder or impossible to maintain.

@anybody: How to proceed here? Probably, I make all the bootstrap changes in an issue fork here: #3410571, so you can see if a setting or submodules are better?

@thomas.frobieter yes I think so. All in one MR so we can see what needs to be changed and decide then, how to do it finally.

Instead we made https://www.drupal.org/project/drowl_paragraphs_bs →