* lisp/ox-linguistics.el: package-case macro
* lisp/ox-linguistics.el: use package-case macro for dispatching on package name
* lisp/ox-linguistics.el: package-case: bugfix: body exprs should be
  spliced in
* lisp/ox-linguistics.el: package-case: clarify docstring
* lisp/ox-linguistics.el: simplify org-linguistics-plain-list