Support `scroll_down` command at the end of a file

[aleksanderkrauze]

In contrast to vim's <C-e> in helix scroll_down will stop when the end of the file is reached. This makes moving the end of a file higher in the terminal not possible using this method. I know that it can be done using align_view_top, but this requires changing one motion to another.

I propose aligning helix's behavior to vim's. I will try providing a PR for that.

[pascalkuthe]

Tgis was accidentialmy fixed by #5420 but #6966 is about restoring that behavior so we have two conflicting issues now. We should either decide to keep the current behavior or decide to fix #6966. Slightly leaning towards keeping the current behavior for simplicity. You can always scroll back up.

[pascalkuthe]

Pl we decided to keep this so this was fixing by #5420