Return the height, in lines, of WINDOW's text area.
WINDOW must be a live window and defaults to the selected one.
The returned height does not include the mode line or header line.
On a graphical display, the height is expressed as an integer multiple
of the default character height. If a line at the bottom of the text
area is only partially visible, that counts as a whole line; to
exclude partially-visible lines, use `window-text-height'.
(fn &optional WINDOW)