emacs: Add package info heading.

emacs: Add package info heading.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
* emacs/guix-info.el: (guix-package-info-heading): New face.
  (guix-package-info-synopsis): Modify face.
  (guix-info-insert-entry-default, guix-package-info-insert-heading,
  guix-package-info-insert-with-heading): New procedures.
  (guix-package-info-heading-params, guix-info-insert-package-function,
  guix-info-insert-installed-function, guix-info-insert-output-function,
  guix-info-insert-generation-function): New variables.