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

[martin rudalics]

> IMO, it's too early to discuss such an option: we don't yet know how
> this feature will end up in Emacs 27 and beyond, because we didn't yet
> have enough user feedback.  What we have now is similar to how other
> programs behave, including highlighting of the space beyond EOL.  Does
> it irritate you in those other programs as well?

I've never seen another program highlighting the newline character the
way we do now.

martin