Use scheme-mode for "csi" interpreter

I like writing CHICKEN Scheme scripts without the .scm extension -
this lets me auto-enable scheme-mode for them.

contrapunctus/cp-lisp.el

@@ -49,6 +49,10 @@
 ;;   ;;  `((,(kbd "C-."))))
 ;;   )
 
+(use-package scheme-mode
+  :defer t
+  :interpreter "csi")
+
 (defun contrapunctus-lisp-copy (arg)
   "Run `whole-line-or-region-copy-region-as-kill' if region is
 active, else `sp-copy-sexp'."