It seems that functions like treesit-next-sibling, treesit-prev-sibling,
treesit-child-by-field-name, treesit-first-child-for-pos, treesit-child-count
were planned to implement, but their addition was postponed.  For this reason
I just commented out them, to be able to easily uncomment when they will
be implemented.  I could push other changes to emacs-29 as well: