patch 9.0.1820: Rexx files may not be recognised

Problem:  Rexx files may not be recognised
Solution: Add shebang detection and improve disambiguation of *.cls
	  files

closes: #12951

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>
diff --git a/runtime/autoload/dist/script.vim b/runtime/autoload/dist/script.vim
index 6163003..fca5dcd 100644
--- a/runtime/autoload/dist/script.vim
+++ b/runtime/autoload/dist/script.vim
@@ -213,6 +213,10 @@
   elseif name =~ '^crystal\>'
     return 'crystal'
 
+    # Rexx
+  elseif name =~ '^\%(rexx\|regina\)\>'
+    return 'rexx'
+
   endif
 
   return ''