This function moves the “insert” and “selection_bound” marks simultaneously.
If you move them in two steps with [method@Gtk.TextBuffer.move_mark], you will temporarily select a region in between their old and new locations, which can be pretty inefficient since the temporarily-selected region will force stuff to be recalculated. This function moves them as a unit, which can be optimized.
where to put the “insert” mark
where to put the “selection_bound” mark