Split the selected window into two side-by-side windows.
The selected window is on the left. The newly split-off window
is on the right, and displays the same buffer. Return the new
window.
If optional argument SIZE is omitted or nil, both windows get the
same width, or close to it. If SIZE is positive, the left-hand
(selected) window gets SIZE columns. If SIZE is negative, the
right-hand (new) window gets -SIZE columns. Here, SIZE includes
the width of the window's scroll bar; if there are no scroll
bars, it includes the width of the divider column to the window's
right, if any.