Add --preview-window=next position (#4801)

Places preview adjacent to input on the list side: above input in the
default layout, below it in --layout=reverse.

  fzf --preview 'cat {}' --preview-window=next

Close #4798

man/man1/fzf.1

@@ -993,9 +993,14 @@ border line.
     \fBdown
     \fBleft
     \fBright
+    \fBnext
 
 \fRDetermines the layout of the preview window.
 
+* \fBnext\fR places the preview window adjacent to the input section, on
+the list side: above the input in the default layout, below the input
+in \fB\-\-layout=reverse\fR.
+
 * If the argument contains \fB:hidden\fR, the preview window will be hidden by
 default until \fBtoggle\-preview\fR action is triggered.