feature request: incorporate shellcheck for linting command blocks

[a-frantz]

This feature will be a complete rip-off from miniwdl. It's my favorite feature of miniwdl check and I want us to copy it.

For completeness: link to shellcheck site

It's been a while since I looked into this, but when I last looked there wasn't a good Rust crate for doing Bash linting. miniwdl shells out to shellcheck and parses the resulting STDOUT for a nice user interface. We are probably going to be stuck doing the same, which isn't ideal bc it's not very robust. But linting command blocks is a great feature I really want us to have, and it's worth getting our hands a little dirty IMO.