bug#13156: dedicated face for parentheses in lisp modes

[Jonas Bernoulli <jonas@bernoul.li>]

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Hello

Could you please add a `paren-face' that could be used in lisp modes
(and possibly others) to dim (or highlight) parentheses.  I am currently
using the attached library to do this but think it would make sense if
this functionality was built-in.

Being very simple `parenface.el' just unconditionally adds an additional
keyword to `[lisp|scheme]-font-lock-keywords' when loaded.

I see two ways of implementing this as part of Emacs.

1. Add the keyword to `[lisp|scheme]-font-lock-keywords-2' directly and
   let `paren-face' inherit from `default'.  Users who actually want
   parens to be dimmed or highlight can then customize the former.

2. Define a local minor mode that adds the additional keywords.

I would prefer (1) but maybe that is not an option because it might
negatively affects font-lock's performance.

If this has to be a mode then I am happy to write it.  I haven't done
so yet because `parenface.el' works fine for me and because I would
actually prefer (1).

  Thanks for considering,
  Jonas


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