Completion buffer resizing can be improved

• Resizing sometimes leads to flicker. It seems that the buffer is displayed briefly and then resized.

• Add an option or example configuration which shows how to achieve a grow-only or fixed size buffer.

• The last line is sometimes cut off due to the pixel resizing. It would be better if the lines are displayed fully. I remember that I had to fight a lot in Vertico to achieve beautiful resizing without cutoffs. I think I could not use fit-window-to-buffer since it felt broken in some ways. In Vertico the problem is of course more severe since only a handful of candidates are shown and minibuffer resizing generally seems to be more problematic due to special casing.