kilo: revisiting the PAGE_UP and PAGE_DOWN keys

Back when I was implementing The PAGE_UP and PAGE_DOWN keys, I offered my own refactored implementation.

I've now just completed the steps to implement vertical scrolling in the text viewer phase of the project, I discover that my implementation no longer has the same effect once there is a file to be scrolled through. Fortunately, there is an easy fix:

    case PAGE_UP: 
        editor_move_cursor(0, -get_screen_height());
        break;
 
    case PAGE_DOWN:
        editor_move_cursor(0, get_screen_height());
        break;

As a side note, I found getting the scrolling code correct was surprisingly fiddly, due at least partially to my slightly different structure and very different naming conventions. I feel like there's some major structural refactoring that needs to be done, but I feel I'm already close to a place where many more changes will get me to the place where I can't map between the original version and my refactored version anymore. So I'm going to hold off on making any big changes at least until I work through the rest of the code.