Add 'backend' header arg to clojure code blocks

[Chris Clark]

This is my first attempt to contribute to org-mode; please let me know if I'm doing something wrong.

Attached is a patch to incorporate some of the changes that were originally presented by Ag Ibragimov here: https://list.orgmode.org/m2y2oimdjf.fsf@gmail.com/. I've been using these changes locally for a long time, and I'm hoping others might benefit!

For example:

   #+header: :backend babashka
   #+begin_src clojure
     (range 3)
   #+end_src

   #+RESULTS:
   : (0 1 2)

0001-ob-clojure.el-Add-a-backend-header-arg-to-clojure-co.patch
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