patch 9.0.2179: no filetype detection for execline scripts

Problem:  no filetype detection for execline scripts
Solution: Add filetype detection for execline

as a prior to adding syntax support for execline (see
https://github.com/djpohly/vim-execline/issues/2), i went ahead and made
the filetype detection for execline scripts.

closes: #13689

Signed-Off-By: Mazunki Hoksaas <rolferen@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/script.vim

@@ -225,6 +225,10 @@ export def Exe2filetype(name: string, line1: string): string
   elseif name =~ '^dart\>'
     return 'dart'
 
+    # Execline (s6)
+  elseif name =~ '^execlineb\>'
+    return 'execline'
+
   endif
 
   return ''