Why not just set a shebang like

    $ cat foo.sh
    #!/usr/bin/env zsh
    echo $SHELL

Which most editors understand.

    $ file foo.sh
    foo.sh: a /usr/bin/env zsh script text executable, ASCII text
    $ vim --clean -c ':set filetype?' foo.sh
    ...
    filetype=zsh

This ALSO ensures that the correct shell is invoked on your script, instead of your current or default shell.

    $ ./foo.sh
    /usr/local/bin/zsh