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

Here's a snippet or screenshot that shows the problem:

This is just a small example, please read More details section below.

#!/bin/sh 
[ -k foo ]

Here's what shellcheck currently says:

No issues detected!

Here's what I wanted or expected to see:

In POSIX sh, -k is undefined.

(Or something similar.)

More details

Shellcheck does not emit warnings for the non-POSIX operators -nt, -ef, -k, -o, <, and > when using a #!/bin/sh shebang or --shell sh.

Shellcheck already emits warnings for non-POSIX syntax (example 1, example 2). It would be useful if shellcheck could also warn for these non-POSIX operators, when the sh shell is specified.