syntax: sh: recognize some shell scripts by their Emacs modeline

This helps with recognizing most files in /usr/share/bash*/completions/,
where --magic is powerless.

syntax/sh.nanorc

@@ -2,6 +2,7 @@
 
 syntax sh "(\.sh|(\.|/)(a|ba|c|da|k|mk|pdk|tc|z)sh(rc|_profile)?|/(etc/|\.)profile)$"
 header "^#!.*/(((a|ba|c|da|k|mk|pdk|tc|z)?sh)|(busybox|env) +sh|openrc-run|runscript)"
+header "-\*-.*shell-script.*-\*-"
 magic "(POSIX|Bourne-Again) shell script.*text"
 comment "#"