Option to specify shell iff shebang and in-file directives are missing

[sparr]

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

I have some files with no shebang and some with. I would like to run the same shellcheck command across all of them, with the dialect set by the shebang in files with one, and to a default otherwise. Unfortunately shellcheck -s dash overrides the shebang in files that have one. I'd like a "soft" equivalent to -s that only applies of there's no shebang or #shellcheck shell= directive in the file.

My use case is testing a bunch of files with a script, without having to code my own shebang-presence checker.