bug#57481: elisp code utilising derived-mode-parent

[Christopher Dimech]

Have seen that  (get 'latex-mode 'derived-mode-parent)  as a normal elisp 
command.
For instance, I cannot run it in "lisp-interaction-mode" or simply make a elisp
function that iterates the result of the expression until reacting "nil" to get
a tree of all parents.

Would be a handy tool to have.