add a vim modeline

This will cause vim (and other properly-extended editors) to read this
as a fish script file, despite its .fish.in extension.

assets/completions/bat.fish.in

@@ -207,3 +207,5 @@ complete -c $bat -l acknowledgements -d "Build acknowledgements.bin" -n __bat_ca
 complete -c $bat -s h -d "Print a concise overview of $bat-cache help" -n __bat_cache_no_excl
 
 complete -c $bat -l help -f -d "Print all $bat-cache help" -n __bat_cache_no_excl
+
+# vim:ft=fish