Add scrolling down past the scroll-off height at the bottom of the buffer

[jrmoulton]

When using scroll via page_down this lets the buffer go beyond just the distance set by scroll off. This is what I expected from vim and is useful because I often use page_down to position the screen to my liking including when I am at the bottom of the buffer. I've currently set this as a configuration feature that defaults to false

If this really should be a configuration feature how should it be named?

[the-mikedavis]

That line prevents page_downing further if the end of the document is visible which is different from the scrolloff. (last_line is the last visible line in the current viewport in this block.) Vim and Kakoune also respect scrolloff when using PageDown at the end of a document.

See #4590 (comment)

I don't think we need a config option for this behavior

[jrmoulton]

Ahh I missed those. That makes sense. I'll go ahead and close this PR