patch 9.0.2062: Janet files are not recognised

Problem:  Janet files are not recognised
Solution: Add filename and shebang detection (without
          adding an extra filetype plugin)

Those are used by the Janet language:
http://www.janet-lang.org

closes: #13400

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>

runtime/autoload/dist/script.vim

@@ -217,6 +217,10 @@ export def Exe2filetype(name: string, line1: string): string
   elseif name =~ '^\%(rexx\|regina\)\>'
     return 'rexx'
 
+    # Janet
+  elseif name =~ '^janet\>'
+    return 'janet'
+
   endif
 
   return ''