bug#39360: Add :extend attribute to show-paren-mode expression face

[Eli Zaretskii]

> Cc: 39360@debbugs.gnu.org
> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Fri, 31 Jan 2020 10:51:08 -0500
> 
> > The newline is inside the parentheses, but the empty space to the 
> > right of the newline is not part of the text.  Why should it be 
> > highlighted?
> 
> For the same reason that the region highlight goes to the edge of the window, 
> I think.

We left the region as it was to avoid too strong outcry.  In general,
region should also not extend, IMO.