Add Actions script to do regular shell format check

I hope @Ricardo Pardini can add some comments here as he did this implementation. IIRC we said that we need this to check everything what comes to PR, while internal functionality only takes care of what is under /lib which is core of framework.

I think we can be happy with detecting errors. Not sure if its worth expanding this further.

Apparently, we have this shell format check script here: https://github.com/armbian/build/blob/main/lib/tools/shellfmt.sh#L66-L75

I guess this is being used locally as a pre-commit or “on demand”.

Btw, when and what do we want to be accomplished here?

• [WHEN] On schedule, run a new Github workflow? (I guess so)

• [WHAT] List files whose formatting differs from shfmt's AND exit the job according to the exit of the shell check?

  • Or do we want to also correct (modify + commit) those files, as part of the workflow?